单选题图示一层框架在垂直荷载作用下的弯矩图，正确的是（　　）。<br/> <img border="0" alt="" src="https://tuku.weimei168.com/uploads/upimg/20251122/18930535_0.jpg"><br/> <img border="0" alt="" src="https://tuku.weimei168.com/uploads/upimg/20251122/18930535_0_1.jpg">