D是闭区域{(x,y)|a2≤x2+y2≤b2}，则 ∫∫ √ x2+y2d= （ ）

A、<img width="74" height="42" alt="" src="https://wendingoss.oss-cn-shenzhen.aliyuncs.com/test/190522/2019052218013824600.png" />

B、<img width="82" height="42" alt="" src="https://wendingoss.oss-cn-shenzhen.aliyuncs.com/test/190522/2019052218013827662.png" />

C、<img width="82" height="42" alt="" src="https://wendingoss.oss-cn-shenzhen.aliyuncs.com/test/190522/2019052218013854805.png" />

D、<img width="80" height="42" alt="" src="https://wendingoss.oss-cn-shenzhen.aliyuncs.com/test/190522/2019052218013828698.png" />