- it is late I shall go home now.-- OK go .A. So; downB. If; upC. Since; aheadD.

问题补充：

---- it is late, I shall go home now.

---- OK, go .

A. So; down

B. If; up

C. Since; ahead

D. Because; out

答案：

A. So; down

B. If; up

(答案→)C. Since; ahead

D. Because; out

解析：

试题分析: 句意：因为晚了，我现在要回家了，好的，请先行。So因为down 向下 If;如果up向上

Since既然ahead 提前 Because;因为out出去，根据t is late, I shall go home now.，可知是告别用语，go ahead先行一步，go down下去，go up 上升，go out 出去，根据语境选C。