图示四种刚架中，哪一种横梁跨中a点弯矩最大？

• A.<img src="https://timg.fentiku.com/97/2016/12/22/twEEBLNRTW.png" title='twEEBLNRTW.png' alt='twEEBLNRTW.png'/>
• B.<img src="https://timg.fentiku.com/97/2016/12/22/aerpzQ3Lth.png" title='aerpzQ3Lth.png' alt='aerpzQ3Lth.png'/>
• C.<img src="https://timg.fentiku.com/97/2016/12/22/VCIlzmvoZo.png" title='VCIlzmvoZo.png' alt='VCIlzmvoZo.png'/>
• D.<img src="https://timg.fentiku.com/97/2016/12/22/q0czZl6Wr5.png" title='q0czZl6Wr5.png' alt='q0czZl6Wr5.png'/>

正确答案：B