图示结构，杆端弯矩 <img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/386a63abcded4945a90e89adbfd35f60.png?x-oss-process=image/sharpen,200"/>为（）<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/194abfe5fa92410facf8e5054bd2d14c.png?x-oss-process=image/sharpen,200"/>