When and why he came here ____ yet.A.is not known B.has not been known并说明原因谢谢

When and why he came here ____ yet.A.is not known B.has not been known并说明原因谢谢
When and why he came here ____ yet.
A.is not known B.has not been known
并说明原因谢谢

When and why he came here ____ yet.A.is not known B.has not been known并说明原因谢谢
b
因为yet是完成时的标志
而且when and why 表示一个整体概念,所以谓语单数

b

选B
yet用于完成时
根据意思是：他何时、为什么来这里还没有人知道

很难解释，你还是去国外呆几年吧