求指导本题解题过程，谢谢您！28化二次积分 =(int )_(0)^1dx(int )_(0)^xf(sqrt ({x)^2+(y)^2})dy 为极坐标下的二次积分,则 I= ()-|||-A. (int )_(0)^dfrac (pi {4)}dtheta (int )_(0)^dfrac (1{cos theta )}f(r)rdr B. (int )_(0)^dfrac (pi {4)}dtheta (int )_(0)^dfrac (1{sin theta )}f(r)rdr A-|||-C. (int )_(0)^dfrac (x{4)}dtheta (int )_(0)^dfrac (1{cos theta )}f(r)dr D. (int )_(0)^dfrac (x{4)}dtheta (int )_(0)^dfrac (1{sin theta )}f(r)dr

求指导本题解题过程，谢谢您！