The owner of a bike shop sells unicycles and bicycles and keeps inventory by counting seats and wheels. One day, she counts 21 seats and 30 wheels. Which equation in slope-intercept form shows that x bicycles and y unicycles together have a total of 30 wheels?
A. y = -2x - 30
B. y = -2x + 30
C. y = 2x + 30
D. y = 2x - 30