下图所示各结构中，所用杆件截面特性的材料均相同，跨度和荷载相同，()结构a点的弯矩最小。

• A.<img src="https://timg.fentiku.com/img/2018-04-09/5acafdb528af6.png" alt="" />
• B.<img src="https://timg.fentiku.com/img/2018-04-09/5acafdb54b996.png" alt="" />
• C.<img src="https://timg.fentiku.com/img/2018-04-09/5acafdb56fbbf.png" alt="" />
• D.<img src="https://timg.fentiku.com/img/2018-04-09/5acafdb59d641.png" alt="" />

正确答案：D