美国的社会结构中，最庞大的是（）。
A.上级阶层
B.中间阶层
C.下级阶层
D.中上阶层
参考答案：B
充量调节阀的控制风压越高，进油口的开度就越（）。
A、大
B、小
C、不变
D、少
Students should by all means ______ their studies though there are a lot of attractions around them. [ ]
A. devote to
B. fix on
C. keep on
D. concentrate on
