一等截面铸铁梁的弯矩图如图所示，设计梁的截面时，最合理的截面应该是图（）<img src="https://nimg.ppkao.com/2018-08/wangjing/2018081610521440109.jpg?sign=6ecc7116ef82fb4ad264c327c258492f&t=62d071b6" /><img src="https://nimg.ppkao.com/2018-08/wangjing/2018081610522251207.jpg?sign=863bf0b9c64282ce72a20ebeccbbdf1f&t=62d071b6" />