Which statement best describes how cholesterol affects cell membrane fluidity?   
A. Cholesterol increases fluidity at high temperature and decreases fluidity at low temperature. 
B. Cholesterol increases fluidity at high temperature and increases fluidity at low temperature. 
C. Cholesterol decreases fluidity at high temperature and decreases fluidity at low temperature. 
D. Cholesterol decreases fluidity at high temperature and increases fluidity at low temperature.