图1-2-57所示结构梁a点处的弯矩是（）。

• A.<img src="https://timg.fentiku.com/javaimportItems/48556df29f8041a698a34d5118931a4f/images/22adaf.png" /><br/>
• B.<img src="https://timg.fentiku.com/javaimportItems/48556df29f8041a698a34d5118931a4f/images/22b4aa.png" /><br/>
• C.<img src="https://timg.fentiku.com/javaimportItems/48556df29f8041a698a34d5118931a4f/images/22bb79.png" /><br/>
• D.<img src="https://timg.fentiku.com/javaimportItems/48556df29f8041a698a34d5118931a4f/images/22c1f9.png" /><br/>

正确答案：A