下列性质中正方形具有而矩形没有的是（　　）

A．对角线互相平分

B．对角线相等

C．对角线互相垂直

D．四个角都是直角

We should try every effort to prevent violence from happening at school, ______ more and more students would drop out of school ______ their personal safety could not be guaranteed.

A．though; if only

B．though; so long as

C．for; now that

D．for; if