he cost, c, of a health club membership is related linearly to its length in months, m. If a 12-month membership costs $260 and an 18-month membership costs $380, which equation can be used to find the cost of a 30-month membership?
A) c - 380 = 1/20(30-18)
B) c - 18 = 20(380-30)
C) c - 380 = 20(30-18)
D) c - 18 = 20(30-12)