a. $\displaystyle\int_0^4\int_0^{\sqrt{r^2-x^2}}\int_0^{\sqrt{r^2-x^2-y^2}}dzdydx$ b. $\displaystyle\int_0^2\int_0^{\sqrt{r^2-y^2}}\int_0^{\sqrt{r^2-x^2-y^2}}dzdxdy$