Betty needs to buy bread and milk for her church picnic. At store A, she would pay $50.73. At store B, she would pay $51.63. Which of the following system of equations represents this situation?

a) 50.73x+51.63y=z
b) 50.73y+51.63x=z
c) 50.73x=51.63y
d) 51.63x=50.73y