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?

Ac - 380 = 1/20(30-18)
Bc - 18 = 20(380-30)
Cc - 380 = 20(30-18)
Dc - 18 = 20(30-12)