A Radar Pulse Sent Out to an Airplane at a Distance

Multiple Choice

A radar pulse sent out to an airplane at a distance of 20.0 km will return as an echo to the source in what time interval? (c = 3.00 x 10⁸ m/s)

A) 33.3 x 10^-6 s
B) 66.7 x 10^-6 s
C) 133 x 10^-6 s
D) 0.033 3 x 10^-6 s