在水源的某处排放污染物，就会污染整个水源，主要原因是 [ ]

A．分子很小

B．分子是可分的

C．分子之间有间隔

D．分子在不断地运动

— What were you doing at this time yesterday?

— I _______ basketball with my friends. [ ]

A. was playing

B. had played

C. have played

D. played