要求防护人员撤回时应鸣示召集信号二长声。
参考答案:错
环境污染越来越严重,人类无能为力去改善已污染的环境。
—Must I go to the shop with you,Mum?
—Err... I can make it myself,Mike. You go with me.
A.mustn’t
B.shouldn’t
C.wouldn’t
D.needn’t