7. Let positive integers $a$, $m$, $n$ satisfy
$$
\sqrt{a^{2}-4 \sqrt{2}}=\sqrt{m}-\sqrt{n} \text {. }
$$

Then the values of $a$, $m$, $n$ ( ).
(A) have one set
(B) have two sets
(C) have more than two sets
(D) do not exist