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

答案是D， 为什么B不对，我认为B是状语从句省略了主谓语they are. 而答案D也符合状语从句省略的条件，能不能变成 if to be呢？那岂不是和B没有区别了？状语从句的省略真让人头痛。

B is not correct.

In when 状语从句, 主谓语they are 是不能省略的. I did not see sentence like"sth.. when to be..."

However, "when to.." seems OK.