请教大全91，

91.Although one link in the chain was demonstrated to be weak, but not sufficiently so to require the recall of the automobile.
(A) demonstrated to be weak, but not sufficiently so to require
(B) demonstrated as weak, but it was not sufficiently so that it required
(C) demonstrably weak, but not sufficiently so to require
(D) demonstrably weak, it was not so weak as to require
(E) demonstrably weak, it was not weak enough that it required
    答案是D，没有问题

Choices A and C entail ungrammatical constructions because they do not produce a sentence that has a main clause with a subject and a verb. In choice B, demonstrated as weak is unidiomatic; also in choices B and C Although and but should not be used together because only one is needed to express the relationship between the ideas. Choice D is best. Choice E is less concise and idiomatic than D; moreover, it is imprecise to say that one link in the chain (the referent of it) actually required the recall. The question is of middle difficulty.

A中为什么说缺少主谓呢，不可以看成one link in the chain was 在but 后省略吗，要是没有but 这题对不对？谢谢

A. Although 引导的是从句，but 引导的是另一个不完整的句子，所以说缺少主谓。一般的， Although ..., 主语 + 谓语 ...