$(7)$ Which statement is correct about sigma$-$delta ADC?

a$.$ The difference of the numbers of $0$s and $1$s in the output indicates the amplitude

of the input signal.

b$.$ The difference between two samples is accumulated.

c$.$ The delta modulation is based on the $8-$bit quantization method.

d$.$ The maximum number of $0$s corresponds to the maximum positive input voltage.