只能在不缺氧的工作环境和低浓度毒物污染环境中使用（）呼吸器。
A、过滤式
B、隔离式
C、自给式
参考答案：A
The reason ______ he gave for not coming was ______ his mother wouldn’t allow him to.
A．why ; that
B．for which ; that
C．why ; because
D．which; that
邮票四周的齿孔有什么用？（）
A、容易撕开
B、美观大方
C、装饰作用
