答案：A
$\begin{array}{cl}
& \sqrt{49a} – \sqrt{25a} \\
= & 7\sqrt{a} – 5\sqrt{a} \\
= & 2 \sqrt{a}
\end{array}$
$\begin{array}{cl}
& \sqrt{49a} – \sqrt{25a} \\
= & 7\sqrt{a} – 5\sqrt{a} \\
= & 2 \sqrt{a}
\end{array}$
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