如图所示杆的扭矩图，正确的扭矩图是（   ）。

A、<span style=";font-family:&#39;Times New Roman&#39;;font-size:14px"><span style="font-family:宋体">图</span>a</span></p><p></p>

B、<span style=";font-family:&#39;Times New Roman&#39;;font-size:14px"><span style="font-family:宋体">图</span>b</span></p><p></p>

C、<span style=";font-family:&#39;Times New Roman&#39;;font-size:14px"><span style="font-family:宋体">图</span>c</span></p><p></p>

D、<span style=";font-family:&#39;Times New Roman&#39;;font-size:14px"><span style="font-family:宋体">都不正确</span></span></p><p></p>