Everyone will go through life's stages of ups and downs, selfrespect plays a key role in the maturity(成熟) of a person.
A. when
B. which
C. as
D. that
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(答案→)A. when
B. which
C. as
D. that
解析：句意：每个人都会经历人生的起起伏伏，这时候自尊对一个人成熟起着关键的作用。这里用了定语从句，先行词是life’s stages，定语从句中不缺主宾表，缺少的是时间状语，用when引导定语从句。