It would seem to be a general truth nothing is as straightforward as it at first seems. A.

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(答案→)A. that
B. when
C. what
D. if
解析：句意：没有什么起初似乎是简单易懂，这似乎是一个一般的真理。It是形式主语，nothing is as straightforward as it at first seems是真正的主语从句，其中nothing是主语，as straightforward as it at first seems是表语。这个主语从句主语，表语齐全，故用that。that只引导名词性从句，不做句子的成分。When表示时间，在句中作状语。what 充当名词性从句的主语，宾语和表语。If 表示是否。故选A。