-- Are these _______ all from No. 2 Middle School?

-- Yes. They often _________ their free time to help the old here.

A．volunteers; volunteers

B．volunteer; volunteers

C．volunteers; volunteer

D．volunteer; volunteer

蒸汽往复泵走不动的可能原因是（）。

A.进汽口和排汽口都被遮住，蒸汽不能进入汽缸

B.进汽口和排汽口都未被遮住，蒸汽不能进入汽缸

C.进汽口和排汽口都被遮住，使蒸汽进入汽缸

D.进汽口被遮住，使蒸汽进入汽缸