问题补充：
They love their daughter so much that they are always ready to do everything they
can her happiness.
A. make sure of
B. be sure of
C. to be sure to
D. to make sure of
答案：
A. make sure of
B. be sure of
C. to be sure to
(答案→)D. to make sure of
解析：
这里考查不定式做目的状语，句意是：他们非常爱他们的女儿，以至于总是准备做能做的任何事来确保她的幸福。所以选D。
