图示桁架，a、b两杆内力为( )。

• A.S<img src="https://timg.fentiku.com/97/2016/12/22/PGpRzSloei.png" title='PGpRzSloei.png' alt='PGpRzSloei.png'/>=0,S<img src="https://timg.fentiku.com/97/2016/12/22/Qd4sJ1E3f0.png" title='Qd4sJ1E3f0.png' alt='Qd4sJ1E3f0.png'/>=0
• B.S<img src="https://timg.fentiku.com/97/2016/12/22/l4PW27Asjy.png" title='l4PW27Asjy.png' alt='l4PW27Asjy.png'/>≠0,S<img src="https://timg.fentiku.com/97/2016/12/22/cgjL8ExzJ2.png" title='cgjL8ExzJ2.png' alt='cgjL8ExzJ2.png'/>≠0
• C.S<img src="https://timg.fentiku.com/97/2016/12/22/L0k8uKt3r6.png" title='L0k8uKt3r6.png' alt='L0k8uKt3r6.png'/>=-S<img src="https://timg.fentiku.com/97/2016/12/22/tsbJOV6o8c.png" title='tsbJOV6o8c.png' alt='tsbJOV6o8c.png'/>
• D.S<img src="https://timg.fentiku.com/97/2016/12/22/0U51CDhHtq.png" title='0U51CDhHtq.png' alt='0U51CDhHtq.png'/>=1/2S<img src="https://timg.fentiku.com/97/2016/12/22/hTUTGWPo3Q.png" title='hTUTGWPo3Q.png' alt='hTUTGWPo3Q.png'/>

正确答案：A