RADAR (Radio Detection and Ranging) uses microwaves, a type of radio wave but usually specific microwave band. Often answer is microwaves. But radio waves also correct. I'll specify microwave as typical answer. Could be radio. Better: microwave. Many exams say microwaves. I'll stick with B.
Explanation:
The acceleration due to gravity at a depth 'd' is given by g' = g(1 - d/R), where R is the radius of the Earth. If we go to the center of the Earth, the depth 'd' becomes exactly equal to R. Substituting d = R into the formula gives g' = g(1 - R/R) = g(1 - 1) = 0. So, gravity is zero at the core.
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