Angela bought gifts for her friends, and each gift cost $10. Let y represent the total cost and x represent the number of gifts. Write an equation to represent the proportional relationship between the total cost and the number of gifts she bought.
Ay = 9x
By = 10x
Cy = 1/10x
Dy = 1/9x
Ay = 9x
By = 10x
Cy = 1/10x
Dy = 1/9x