What is the average of $2^{15}$ and $2^{25}$ ?

Question

$\begin{array}{1 1} (a)\; 2^{40} \\ (b)\; 2^{20} \\ (c)\; 2^{13}+2^{23} \\ (d)\; 2^{12} + 2^{20} \end{array}$