To study a foreign language,（）

A.a good dictionary should be bought

B.buying a good dictionary is necessary

C.you should buy a good dictionary

D.it is necessary that you will buy a good dictionary

收轮后为什么要将起落架手柄放回到"中立（OFF）"位（）

A.这样才能使红色的起落架指示灯熄灭

B.这样可以使液压从起落架系统中释放掉

C.这样才能使轮舱门关闭

D.这样可以使起落架的控制系统复位