Solve the Problem $\$ 100$ Per Month Payment For $2 \frac { 1 } { 2 }$

Solve the problem.
-Bill makes a $\$ 100$ per month payment for $2 \frac { 1 } { 2 }$ years to pay off a $\$ 2500$ loan. What was the add-on interest rate?

A) $10 \%$
B) $8 \%$
C) $12 \%$
D) None of the above is correct.