问题补充：
He devoted most of his time to researching into space, believing his hard work would sooner or later.
A. pay back
B. pay for
C. pay off
D. pay up
答案：
A. pay back
B. pay for
(答案→)C. pay off
D. pay up
解析：考查短语辨析，pay back偿还，pay for为了什么付款，pay off还清，得到回报，pay up 付清，句意：他把他的大部分时间献身于研究太空，相信他的努力工作迟早会得到回报的。故选C。
