题目内容：

When and why he came here () yet?A is not known B are not known C has not known D have not been

优质解答

你好,应该是：
A
翻译为 他何时为什么来这儿还不清楚.
句子的主语是whne and why he came here,是一件事
所以为单数
如果用have,应该用被动
所以只能选A
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