题目

先化简，再求值：，其中.  答案：解：原式=            =            =                                                                                                7分            当时            原式=解析:略  －It’s difficult for the village children to cross the river to get to school． －We think a bridge _________ over the river． [　　] A． was built B． should be built C． has been built D． is being built