When and why he came here ________ yet.A.is not known B.are not known C.has not known D.have nWhen and why he came here ________ yet.A.is not known B.are not known C.has not known D.have not been那个正确,为什么
问题描述：
When and why he came here ________ yet.A.is not known B.are not known C.has not known D.have n
When and why he came here ________ yet.
A.is not known B.are not known
C.has not known D.have not been
那个正确,为什么
答
A.is not known
When and why he came here
这是一个主语从句,看成是单数的主语