While he was investigating ways to improve the telescope, Newton made ______ discovery which completely changed ______ man’s understanding of colour.

A．a; 不填

B．a; the

C．不填; the

D．the; a

压气机清洗的目的是：（）.

A.恢复压气机性能，降低燃油消耗率

B.保持压气机清洁

C.清除灰尘，赃物

D.增加空气流量