图示结构中，杆a的轴力N为下列何值？

• A.N<img src="https://timg.fentiku.com/97/2016/12/22/Nf6dfbrzjC.png" title='Nf6dfbrzjC.png' alt='Nf6dfbrzjC.png'/>=0
• B.N<img src="https://timg.fentiku.com/97/2016/12/22/NFroDlhphh.png" title='NFroDlhphh.png' alt='NFroDlhphh.png'/>=P/2
• C.N<img src="https://timg.fentiku.com/97/2016/12/22/VTf8RDgD9w.png" title='VTf8RDgD9w.png' alt='VTf8RDgD9w.png'/>=P
• D.N<img src="https://timg.fentiku.com/97/2016/12/22/6K31z5MvHc.png" title='6K31z5MvHc.png' alt='6K31z5MvHc.png'/>=3P/2

正确答案：D