常用的大洋航线有（）。
A.大圆航线
B.混合航线
C.恒向线航线
D.以上都是
参考答案：D
— What's the matter with Andy ? He doesn't look very well.
— He has a headache because he ______ too long; he ought to have stopped reading.[ ]
A. had read
B. is reading
C. was reading
D. has been reading
从所给的四个选项中，选择最合适的一个填入问号处，使之呈现一定的规律性（）
A.A
B.B
C.C
D.D
