单选题图示桁架，杆件1的内力是（　　）。<br/> <img border="0" alt="" src="https://tuku.weimei168.com/uploads/upimg/20251122/18930628_0.jpg"><br/> A.P<br/> B.<img border="0" alt="" src="https://tuku.weimei168.com/uploads/upimg/20251122/18930628_0_1.jpg">P<br/> C.2P<br/> D.0