Constant element refers to$\_\_\_\_\_\_\_\_\_\_\_$

a$.$

an element, which occurs in every cycle of an operation.

b$.$

an element, which does not occur in every cycle of an operation.

c$.$

an element where basic time and the work content stay the same or are unchanging.

d$.$

an element where basic time and the work content varies or changes from one element to another.