$\sqrt{8}+\sqrt{18}=$
\[\text{(A)}\ \sqrt{20}\qquad\text{(B)}\ 2(\sqrt{2}+\sqrt{3})\qquad\text{(C)}\ 7\qquad\text{(D)}\ 5\sqrt{2}\qquad\text{(E)}\ 2\sqrt{13}\]
$\sqrt{8} = 2  \sqrt{2}$
$\sqrt{18} = 3  \sqrt{2}$
So adding the two terms we get $2  \sqrt{2} + 3  \sqrt{2} = 5  \sqrt{2}$ , which corresponds to answer choice $\boxed{\textbf{(D)}}$ .