For the type of digital modulation called frequency shift keying $($FSK$),$ which of the following is correct?

a$.$ Ones and zeros are represented by frequencies that are raised or lowered slightly from the carrier frequency

b$.$ Ones and zeros are represented by two different amplitudes imposed on the carrier wave

c$.$ Ones and zeros ard represented by slightly advanced or retarded phase compared to thr original carrier wave

d$.$ Ones and zeros are represented by a combination of different amplitudes and frequency shifts applies to the carrier