请英语帝,讲解这道题when and why he came here _______yet.a\is not knownb\are not knownc\has not knownd\have not know

请英语帝,讲解这道题when and why he came here _______yet.a\is not knownb\are not knownc\has not knownd\have not know
请英语帝,讲解这道题
when and why he came here _______yet.
a\is not known
b\are not known
c\has not known
d\have not know

请英语帝,讲解这道题when and why he came here _______yet.a\is not knownb\are not knownc\has not knownd\have not know
我不是英语帝……但可保证答案
A
when and why he came here “何时和为什么” 是一件事情
动词用单数的
C应该用被动形式has not been known,排除