图示结构中杆a的内力N(kN)应为下列何项?

• A.N<img src="https://timg.fentiku.com/97/2016/12/22/ekBGselh1f.png" title='ekBGselh1f.png' alt='ekBGselh1f.png'/>=0
• B.N<img src="https://timg.fentiku.com/97/2016/12/22/R9XZ47ZDEx.png" title='R9XZ47ZDEx.png' alt='R9XZ47ZDEx.png'/>=10(拉力)
• C.N<img src="https://timg.fentiku.com/97/2016/12/22/Jb3CIr8phy.png" title='Jb3CIr8phy.png' alt='Jb3CIr8phy.png'/>=10(压力)
• D.<img src="https://timg.fentiku.com/97/2016/12/22/Oa3AvHDcaX.png" title='Oa3AvHDcaX.png' alt='Oa3AvHDcaX.png'/>

正确答案：B