Yasmin and Zippy entered an apple pie eating contest. Together, the two ate 20 slices of pie, and Yasmin ate 4 more slices than Zippy did. Which of the following systems of equations represents this situation in terms of y, the number of slices of pie Yasmin ate, and , the number of slices of pie Zippy ate?
A. y+z=20
y+4=z
B. y+z=20
z+4=y
c. y+z=20
y=4z