问题补充：
They love their daughter so much that they are always ready to do everything they can her happiness.
A. to make sure of
B. make sure of
C. be sure of
D. to be sure of
答案：
(答案→)A. to make sure of
B. make sure of
C. be sure of
D. to be sure of
解析：make sure of确保；be sure of确信，对……有把握。句意：他们如此爱他们的女儿以至于他们总是愿意做他们能做的一切来确保她的幸福。所以选A，不定式作目的状语。
