The title below that best expresses the ideas of this passage is _________.

A．compel the subjects to separate major information from minor details
B．put the subjects through increasingly complex tests
C．check the effectiveness of nicotine on smokers
D．register the prompt responses of the subjects

A．to test whether smoking has a positive effect on the mental capacity of smokers
B．to show how smoking damages people's mental capacity
C．to prove that smoking affects people's regular performance
D．to find out whether smoking helps people's short-term memory