Fitness Fun charges a $100 signup fee plus $15 per month. Weightlifting World charges $30 per month. Which system models this situation?

A. y = 15x + 100
y=30x

B. y = 30x + 100
y = 15x

C. y = 30
y = 100 + 15

D. y = 100x + 15
y = 30x