The problem is sometimes when we are given something on a constant basis, we start to take them for granted and forget being thankful for what we already have.
A. that
B. how
C. where
D. which
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(答案→)A. that
B. how
C. where
D. which
解析：A 考察名词性从句。主语是The problem，is是系动词，后面接表语从句。从句中有包含有when引导的时间状语从句，由于从句句意通顺，成分完整，应该使用that引导，that不充当成分，只做引导词。故A正确。