2. A store sells a certain product and makes a profit of $m$ yuan per item, with a profit margin of $20\%$ (profit margin $=\frac{\text{selling price} - \text{cost price}}{\text{cost price}}$). If the cost price of this product increases by $25\%$, and the store raises the selling price so that it still makes a profit of $m$ yuan per item, what is the profit margin after the price increase?
(A) $25\%$
(B) $20\%$
(C) $16\%$
(D) $12.5\%$
2.C.

Let the original purchase price be $a$ yuan, and the profit margin after the price increase be $x \%$, then $m=a \cdot 20 \% = a(1+25 \%) \cdot x \%$.
Solving for $x \%$ gives $x \% = 16 \%$.