i'm afraid this chair is not comfortable____.A.to sit B.to sit on C.to be sat D.to be sat on为什么选B不选D?

i'm afraid this chair is not comfortable____.
A.to sit B.to sit on C.to be sat D.to be sat on
为什么选B不选D?

这里是动词不定式做后置定语.动词不定式做后置定语时可以看成是动宾结构,即动词不定式和前面的名词形成动宾结构,也就是sit on this chair,而不是be sat on this chair,另外还要注意动词和后面所搭配的介词或副词.例如;
I want a pen to write with.
There is no room to live in.
He is the right man I want to talk about.