2. A box contains 25 tiles that are either triangular or square, with a total of 84 sides. How many square tiles are in the box?
(A) 3
(B) 5
(C) 7
(D) 9
(E) 11
2. D.

Let there be $a$ triangular tiles and $b$ square tiles in the box.
According to the problem, we have
$$
\left\{\begin{array}{l}
a+b=25, \\
3 a+4 b=84
\end{array} \Rightarrow(a, b)=(16,9)\right.
$$