A

$\int_0^3dy\int_{y^2}^{3y}f(x,y)dx$

B

$\int_{x^2}^{3x}dx\int_0^3f(x,y)dy$

C

$\int_{y^2}^{3y}dx\int_0^3f(x,y)dy$

D

$\displaystyle\int_0^9dy\int_{y/3}^{\sqrt y}f(x,y)\,dx$