what is the smallest number of science students who could be studying biology

If 20% of the science students at central high study only physics, 25% study only chemistry…

If 20% of the science students at central high study only physics, 25% study only chemistry and the rest study only biology, what is the smallest number of science students who could be studying biology?

A. 4
B. 9
C. 11
D. 15