Questions pts$]$ A nonconducting spherical hollow shell with inner radius a and outer radius $b$ has positive charge $Q$ uniformly distributed throughout the shell. A particle with charge $q$ is then placed at the center of the cavity. The magnitude of the electric field at a point at the surface of the conducting shell, such that $r=b$ from the center, is:

A$)\frac{Q}{4{}_0{a}^2}$

B$)\frac{q+Q}{4}{}_0{b}^2$

C$)\frac{q}{4}{}_0{r}^2$

D$)0$

E$)\frac{q}{4{}_0{r}^2}+\frac{Q}{4{}_0{r}^2}(\frac{r^2-a^2}{b^2-a^2})$