霜冻常出现于深秋至第二年早春晴朗的夜晚,原因是[ ]
A、大气对地面辐射的反射作用强
B、大气对地面辐射吸收作用强
C、大气的逆辐射作用弱
D、大气的保温作用强
答案:C
两个相反数之和为______.
---- Have you been here before?
---- This is the second time I ______to the Great Wall.
A.comes
B.have been
C.came
D.had come