Mark partially drained his pool to clean it. At 1:00 P. M., he started to refill it. At 5:00 P. M., the pool had 10,400 gallons of water. At 9:00 P. M., the pool had 13,800 gallons of water. If filling the pool is modeled by a linear function, how many gallons of water still remained in the pool when he first started to refill it?

A) 6,150
B) 6,500
C) 7,000
D) 7,150