It can be inferred from the statement about mother chimpanzees and their young (lines 21-23) that young chimpanzees have difficulty ______.

A．be an indicator of cognitive ability
B．vary among individuals within a species
C．be related to food consumption
D．correspond to levels of activity

A．to change shifts at longer intervals
B．to have longer shifts
C．to arrange for some people to work on night shifts only
D．to create better living conditions for night workers