A garrison of 700 men is provisioned for 30 days at the rate of `2 1/2` kg per day per man. If 100 men had left the farrison, then the provisions would have lasted or x days at the rate of ` 3 1/2` kg per dya per man. Find the value of x . Choose the correct answer from the following options:
A. 18
B. 20
C. 25
D. 15