题目

The work we have done has brought great benefits.We are happy that our efforts are not ________. A．in use                          B．in need C．in vain                         D．in order 答案：C如果x=1是方程x2+ax+1=0的一个根，那么a的值是（ ）A．0 B．1 C．2 D．﹣2