地球上蚊虫到处都有，只有（）未发现。
A.西藏
B.东北
C.上海
D.南极洲
参考答案：D
Only when your identity has been checked ____________. [ ]
A. you are allowed in
B. you will be allowed in
C. will you allow in
D. will you be allowed in
砂轮机的作用是什么？
