在TCP/IP参考模型中，提供无连接服务的传输层协议是( )。

A．UDP

B．TCP

C．ARP

D．OSPF

____ for the fact that she got hit by a car and broke her leg on her way to school, she might have passed

the exam.[ ]

A. Had it not been

B. Hadn't it been

C. Was it not

D. Were it not