问题补充：
--- Believe me, all your efforts will some day. --- Thanks for your encouragement.
A. pay back
B. pay down
C. pay off
D. pay for
答案：
A. pay back
B. pay down
(答案→)C. pay off
D. pay for
解析：词义辨析。pay back偿还，报复；pay down即时支付，付定金；pay off付清，取得成功；pay for为……付款。句意：相信我，你的所有努力有一天一定会获得成功。谢谢你的鼓励。所以选C。
