A pool is being drained at a constant rate. The pool still contains 9,000 gallons of water after draining for 20 minutes at a constant rate of 50 gallons of water per minute. Which equation represents the gallons of water, y, remaining in the pool after it has been draining for x minutes?
a) y=9000−50x
b) y=9000+50x
c) y=50x−9000
d) y=50x+9000
