Last season two running backs on the Steelers football team rushed a combined total of 1550 yards. One rushed 4 times as many yards as the other. Let x and y represent the number of yards each individual player rushed. Which system of equations could be used?
a. x+y=1550
y=4x
b. y-x=1550
y=4x
c. x=y=1550
y=x+4
d. y=1550+x
y=x+4