Teachers should _____ their students to practice _____ English often.

A.encourage; speaking   B.let; speak    C.keep; speak

防止支气管扩张急性感染关键在于

A．加强呼吸道痰液引流

B．选择广谱抗生素

C．使用支气管扩张药

D．祛痰药

E．家庭氧疗