When we went past her house, we heard her ____ in her room.

A．dance

B．dancing

C．sing

D．singing

于家用空调器（冷气机）运转时进行充注，应在何处注入制冷剂（）

A.高压侧

B.低压侧

C.高低压两侧同时注入

D.以上皆非