下列  是C语言的有效标识符?

A、<span style="line-height: 18.88888931274414px;">_No1</span>

B、<span style="line-height: 18.88888931274414px;">No.1</span>

C、<span style="line-height: 18.88888931274414px;">12345</span>

D、<span style="line-height: 18.88888931274414px;">int</span>