79. A store sells a certain product and makes a profit of $m$ yuan per item, with a profit margin of $20\%$ (profit margin $=\frac{\text{selling price - purchase price}}{\text{purchase price}} \times 100\%$). If the purchase 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. $12.5\%$
Answer: C