The correct answer is $\boxed{\text{B. 2 : 1}}$.
Let $x$ be the amount that A and B originally invested in the business. Then, A’s share of the profit was $\frac{3}{5}x$ and B’s share was $\frac{2}{5}x$. When C is admitted, the new total share is $1$ share. Therefore, A and B must give up a total of $\frac{1}{4}$ share to C. This means that A gives up $\frac{3}{10}$ share and B gives up $\frac{2}{10}$ share. The sacrificing ratio is the ratio of the shares given up by A and B, which is $\frac{3}{10} : \frac{2}{10} = 3 : 2$.
Here is a step-by-step solution:
- Let $x$ be the amount that A and B originally invested in the business.
- A’s share of the profit was $\frac{3}{5}x$ and B’s share was $\frac{2}{5}x$.
- When C is admitted, the new total share is $1$ share.
- Therefore, A and B must give up a total of $\frac{1}{4}$ share to C.
- This means that A gives up $\frac{3}{10}$ share and B gives up $\frac{2}{10}$ share.
- The sacrificing ratio is the ratio of the shares given up by A and B, which is $\frac{3}{10} : \frac{2}{10} = 3 : 2$.