What is the average (arithmetic mean) of $2^{10}$ and $2^{20}$ ?

Question

$\begin{array}{1 1} (a)\;2^9 + 2^{10} \\ (b)\; 2^9 + 2^{21} \\ (c)\; 2^{11} + 2^{21} \\ (d)\; 2^9 + 2^{19} \end{array}$