问题补充：
They were determined to carry out the plan at first, but then we persuade them to change their minds.
A. would
B. could
C. were able to
D. had to
答案：
A. would
B. could
(答案→)C. were able to
D. had to
解析：考查be able to用法。Be able to do sth表示经过努力做成某事。句意：起初他们决定执行那个计划，但是后来，我们说服了他们改变了主意。
