when to be ...求教

Fibers of hair and wool are not continuous and must normally be spun into thread______woven into textile fabrics.

(a)as are they

(b)when to be

(c)that they are

(d)if they are to be

answer is (D)....why B is wrong ? thanks a lot!!

but : When I was only three years old, my father took me to a circus.

we can omit "I was"--->When only three years old, my father......

why this sentence we can't choose  "when(they are ) to be "......再求教linlin315

when to be的省略结构，是指状语从句的省略。而且要求从句的主语跟主句的主语完全一致，而且逻辑清楚，而且一般指代离他最近的名词。其实这个好像不应该是机考的考点。所以在托福里，一般都是连接完整的句子，只有在没有其他干扰的时候才会出现你说的状况。

这里如果用B，会造成到底是定语从句还是状语从句的迷惑，也就是说到底修饰thread还是主句主语，所以只有完全表达才会比较清楚。