Exercise is __ as any other to lose unwanted weight .A as useful a way B as a useful way

题目

Exercise is __ as any other to lose unwanted weight .A as useful a way B as a useful way
为什么选A不选B 求详解

答案

as...as...是一个固定词组
中间肯定是形容词或者副词
表示和.一样
b中的形容词做了名词的定语了,是不行滴