when and why he came here _____ yet.A.is not knownB.are not knownC.has not known

when and why he came here _____ yet.A.is not knownB.are not knownC.has not known
when and why he came here _____ yet.
A.is not known
B.are not known
C.has not known

when and why he came here _____ yet.A.is not knownB.are not knownC.has not known
When and why he came here 作为一个整体,
谓语动词用单数形式..
句意:他何时和为何来这的原因还未知.!'
选AAAAAAAA.

A
When and why he came here是一个整体，作主语看

A, 这是现在未来式...

A

a

A.is not known
被动式，主语是主语从句
B.are not known
应该是is not known
C.has not known
应该是has not been known.

A

A