The key----the problem is to meet the demand made by the customers.A.to solve B.to solving

选B.to solving
the key to + 动名词那怎么解释the time to go等用to do 的呢？the key to doing sth.中, to 是作为介词形式.the time to do sth.中, to do 是动词不定式.可这两个不是很像吗，那怎么才能知道这个词后面的to是介词还是不定式？the key to doing sth.the time to do sth.以上都为固定搭配.要判断to是介词还是不定式, 就根据语法/语境去判断.