Which statement is an example of ceteris paribus?

An example of ceteris paribus is the statement

A$.$ To study the effect of increased private study time, increase the time spent in the classroom

B$.$ To find the effect on the number of workers hired, let the capital they use increase

C$.$ To study the effect of increased tuition of student loans, keep the interest rate constant

D$.$ To study the effect of a rise in the oil price, keep all prices constant