D是闭区域{(x,y)|a2≤x2+y2≤b2}，则 ∫∫ √ x2+y2d= （） A、<img width="74" height="42" alt="" src="/lqwvjeImg/69/694b7ce8bbe94d26515619ef432bd448.png" /> B、<img width="82" height="42" alt="" src="/lqwvjeImg/59/592266e66a713cda5e319db388b12b49.png" /> C、<img width="82" height="42" alt="" src="/lqwvjeImg/45/453de64de7595796a6aca94bad4d7a5e.png" /> D、<img width="80" height="42" alt="" src="/lqwvjeImg/13/139f2f906a391a43448019b74f27e0d0.png" />