"Considering the echo principle, which of the following points cannot usefully assist in a preliminary understanding of radar detection?
A.The echo will become weak after being blasted.
B.The chance of detecting an echo depends on the loudness and duration of the secondary blasts.
C.Short blasts are required if echoes from close targets are kept received.
D.A sufficiently long interval between blasts is required to permit the reception of return echo."
正确答案是B