Which of the following is/are correct with respect to frequency modulation

Options

A. the amplitude of the modulated wave varies as the frequency of carrier wave
B. the frequency of the modulated wave varies as the amplitude of the modulating wave
C. modulation index of frequency modulation is
`mu=("change in frequency")/("original frequency")`
D. frequency modulation vary the carrier frequency within some small range about its original value.

Explanation

In frequency modulation, the frequency of the modulated wave varies as the amplitude of the modulating wave, modulation index is
`("Change in frequencuy")/("Original frequency"), i.e., (Deltaf)/(f)`. In frequency modulation, frequency of carrier wave is varied within small ragne about original frequency.