34   A wildlife expert predicts that the reintroduction of the caribou into northern Minnesota would fail if the density of the timber wolf population in that region is more numerous than one wolf for every 39 square miles.

(A) would fail if the density of the timber wolf population in that region is more numerous than

(B) would fail provided the density of the timber wolf population in that region is more than

(C) should fail if the timber wolf density in that region was greater than

(D) will fail if the density of the timber wolf population in that region is greater than（D）

(E) will fail if the timber wolf density in that region were more numerous than

答案B 中为什么provided 在这里引导条件句是错的（其他错误先不谈吧），只可用if？

答案解释是：B uses the pretentious and illogical word provided for if in a conditional clause after a negative idea (would fail).

是不是因为fail为否定意义才用provided，provided和if的区别是什么？谢谢

多谢！

以下来自longman,摘出来加深一下印象：

provided:只要，如果

conjunction used to say that something will only be possible if something else happens or is done

eg:I don't mind Guy coming with us , provided he pays for his own meals.