图示哪一种结构不属于拱结构？

• A.<img src="https://timg.fentiku.com/97/2016/12/22/w0d0piDyuG.png" title='w0d0piDyuG.png' alt='w0d0piDyuG.png'/>
• B.<img src="https://timg.fentiku.com/97/2016/12/22/qQsp0M68jd.png" title='qQsp0M68jd.png' alt='qQsp0M68jd.png'/>
• C.<img src="https://timg.fentiku.com/97/2016/12/22/7S4K2SY8JF.png" title='7S4K2SY8JF.png' alt='7S4K2SY8JF.png'/>
• D.<img src="https://timg.fentiku.com/97/2016/12/22/cuxzjEX3sQ.png" title='cuxzjEX3sQ.png' alt='cuxzjEX3sQ.png'/>

正确答案：B