If we do not deal with ________ problem caused by air pollution, our planet may no longer be ________ healthy place for us.

A．a; the

B．a; a

C．the; a

D．the; the

（）焊件焊缝内部缺陷常用超声波检验来擦测。

A.大厚度

B.中厚度

C.薄

D.各种