<p><span style="font-size: 13.008px;">The correct answer is (B) "as a result of". </span></p><p><span style="font-size: 13.008px;">This phrase is used to indicate the cause or reason for something. In this case, the population growth is the result of high birth rates and low death rates. The phrase "as a result of" is commonly used to show cause and effect. </span></p><p><span style="font-size: 13.008px;">The other options are not suitable in this context: </span></p><p><span style="font-size: 13.008px;">(A) "regarding" means about or concerning something, but it does not indicate a cause and effect relationship. </span></p><p><span style="font-size: 13.008px;">(C) "on behalf of" means in place of or for the benefit of someone else, which does not fit the sentence. </span></p><p><span style="font-size: 13.008px;">(D) "assuming that" means making a hypothesis or supposition, which is not appropriate in this context.</span><br></p>