Wemustapplywhatwehavelearnedtoourdailyworkbecauseinnocase_A__frompractice.
AshouldtheorybeseparatedBtheoryshouldbeseparated
CshouldtheoryseparateDtheoryshouldseparate
请详细解释下为什么选A.
顺便分析下这个句子的句子成分
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