<span id="docs-internal-guid-23bc379c-7fff-a5b6-e5b3-a8009563ac2a"><p dir="ltr" style="line-height:1.2;text-align: justify;margin-top:0pt;margin-bottom:0pt;"><span style="font-size: 14pt; font-family: "Times New Roman", serif; color: rgb(0, 0, 0); background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-alternates: normal; font-variant-position: normal; white-space-collapse: preserve;">In the question, two clauses are connected only with a comma without any coordinating conjunction. This suggests that the question is a complex sentence. Preceded by an adverb clause starting from the blank and ending before the comma, the main clause is “WhatsApp’s messages are encrypted.” In other words, the blank should be filled with a subordinating conjunction that starts an adverb clause. </span></p><br><p dir="ltr" style="line-height:1.2;text-align: justify;margin-top:0pt;margin-bottom:0pt;"><span style="font-size: 14pt; font-family: "Times New Roman", serif; color: rgb(0, 0, 0); background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-alternates: normal; font-variant-position: normal; white-space-collapse: preserve;">All the four choices can serve this function. The key to identifying the correct answer is the context. The context suggests that Choice (D) </span><span style="font-size: 14pt; font-family: "Times New Roman", serif; color: rgb(0, 0, 0); background-color: transparent; font-style: italic; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-alternates: normal; font-variant-position: normal; white-space-collapse: preserve;">Whereas</span><span style="font-size: 14pt; font-family: "Times New Roman", serif; color: rgb(0, 0, 0); background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-alternates: normal; font-variant-position: normal; white-space-collapse: preserve;"> is the correct answer since this subordinating conjunction shows a contrast between the message in the adverb clause and that in the main clause. </span></p><br><p dir="ltr" style="line-height:1.2;text-align: justify;margin-top:0pt;margin-bottom:0pt;"><span style="font-size: 14pt; font-family: "Times New Roman", serif; color: rgb(0, 0, 0); background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-alternates: normal; font-variant-position: normal; white-space-collapse: preserve;">Choice (A) </span><span style="font-size: 14pt; font-family: "Times New Roman", serif; color: rgb(0, 0, 0); background-color: transparent; font-style: italic; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-alternates: normal; font-variant-position: normal; white-space-collapse: preserve;">Because</span><span style="font-size: 14pt; font-family: "Times New Roman", serif; color: rgb(0, 0, 0); background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-alternates: normal; font-variant-position: normal; white-space-collapse: preserve;"> starts an adverb clause that explains a cause. Choice (B) </span><span style="font-size: 14pt; font-family: "Times New Roman", serif; color: rgb(0, 0, 0); background-color: transparent; font-style: italic; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-alternates: normal; font-variant-position: normal; white-space-collapse: preserve;">Until </span><span style="font-size: 14pt; font-family: "Times New Roman", serif; color: rgb(0, 0, 0); background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-alternates: normal; font-variant-position: normal; white-space-collapse: preserve;">shows a particular time at which something continues and stops. Choice (C) </span><span style="font-size: 14pt; font-family: "Times New Roman", serif; color: rgb(0, 0, 0); background-color: transparent; font-style: italic; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-alternates: normal; font-variant-position: normal; white-space-collapse: preserve;">Unless</span><span style="font-size: 14pt; font-family: "Times New Roman", serif; color: rgb(0, 0, 0); background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-alternates: normal; font-variant-position: normal; white-space-collapse: preserve;"> is used to say that something will happen or be true if something else does not happen or is not true. All the three choices don’t fit the context and thus are incorrect. </span></p><div><span style="font-size: 14pt; font-family: "Times New Roman", serif; color: rgb(0, 0, 0); background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-alternates: normal; font-variant-position: normal; white-space-collapse: preserve;"><br></span></div></span>