what their friendships are based upon, the important thing to remember is both of them are

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A. Regardless that
(答案→)B. Regardless of, that
C. Though, /
D. Despite, what
解析：考查固定词组和表语从句。Regardless of/despite不管；尽管；后面接名词或者名词短语；though后面接句子表示尽管；第二空that引导表语从句，that在句中不充当任何成分，只起引导作用。句意：不管他们的友谊是建立在什么之上的，需要记住的就是他们是朋友。