2. For a rhombus with a side length of 5, one of its diagonals is no longer than 6, and the other is no less than 6, then the maximum value of the sum of the lengths of the two diagonals is
A. $10 \sqrt{2}$
B. 14
C. $5 \sqrt{6}$
D. 12
Let the lengths of the two diagonals be $2a, 2b (a \leqslant b)$, by $a^{2}+b^{2}=25 \Rightarrow b=\sqrt{25-a^{2}}$, thus $a+b=a+\sqrt{25-a^{2}}(00 \Rightarrow f(x)$ is monotonically increasing $\Rightarrow f(x)_{\max }=f(3)=7$, hence $2a+2b \leqslant 14$. Therefore, the answer is $B$.