In a fort there is sufficient food for 600 men for a month. If 400 more men arrive the fort then how long the food is sufficient for now?

In a fort there is sufficient food for 600 men for a month. If 400 more men arrive the fort then how long the food is sufficient for now?

A. 20 days
B. 18 days
C. 28 days
D. 36 days