Which of the following statement is correct for Time Division Multiplexing $($TDM$)$:

Group of answer choices

c$)$ If the time slots are preassigned and fixed, such as the Synchronous TDM$,$ when a connected device is not transmitting, the corresponding slot is empty.

d$)$ All of above.

a$)$ Each user takes a fraction of the time. Sampling rate and the number of bits used for representing the samples determine the quality;

b$)$ In synchronous TDM systems, the multiplexer picks each input in turn to fit a predefined time slot;