—Why did you leave the lights burning?

—Heavens! I guess I     to turn them off.

A．would forget

B．had forgot

C．forgot

D．have forgot

按照国际标准化组织制定的开放系统互连参考模型，实现端用户之间可靠通信的协议层是（）。

A．应用层

B．会话层

C．传输层

D．网络层