答案：B. why
解析：the reason that 后接具体原因,如:the reason that he was ill yesterday is such a bad excuse.意为:那个他昨天生病的理由真是个糟糕的借口.
the reason why 后接对这个理由的修饰,或者是现在需要解释的一个状况.如:the reason why he was late is that he was ill. 在这里,he was late是一个需要解释的情况,所以用 the reason why ,这里,用到一个句型:the reason why ... is that... ,依照上面示例可知用法.
翻译：那是我来这的主要原因.
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