The Calls of Echolocating Bats Are "Frequency Modulated," Meaning That

The calls of echolocating bats are "frequency modulated," meaning that

A) each pulse repeatedly rises and falls in frequency.
B) the rate of pulse production declines as the bat nears an object.
C) the frequency of the pulse is high at the beginning and drops toward the end.
D) the transmission-to-reception time increases as the bat nears an object.
E) None of the choices are correct