A solid sphere of radius R has a charge Q distributed in its volume with a charge density\[\rho =k{{r}^{a}}\], where k and an are constants and r is the distance from its center. If the electric field at \[r=\frac{R}{2}\] is \[\frac{1}{8}\] times that at \[r=R\], the value of a is.

• A. 3
• B. 5
• C. 2
• D. both [a] and [b]