Submitted by admin on Mon, 04/21/2014 - 21:51
definition:
The emission of energy as electromagnetic waves or as moving subatomic particles, especially high-energy particles which cause ionization
sentence:
We now know that invisible forces do control some things: gravity, radiation, electricity.
Because it uses sound waves instead of radiation, ultrasound is safer than X-rays
difficulty:
medium
toefl frequency:
2
toefl classification: