在正2004边形A₁A₂…A₂₀₀₄各顶点上随意填上1，2，…501中的一个数．试证明：一定存在四个顶点满足如下条件：

（1）这四个顶点构成的四边形为矩形；

（2）此四边形相对两顶点所填数之和相等．

After a 5-year absence, we returned to our mother school, and not until then       it was such a beautiful school.

A．we realized

B．had we realized

C．we had realized

D．did we realize