Melanie buys 2 shells for $0.80. Rosi buys 3 shells for $1.20. Carlos buys 4 shells for $1.60. Use the graph to determine whether there is a proportional relationship between the number of shells and their cost. If so, what does the constant of proportionality mean in this situation? Use the drop-down menus to show and explain your answer. Gift shop shells graph showing cost in dollars on the y axis and number of shells on the x axis. A line begins at the origin and passes through the points 2 comma 80 cents, 3 comma 1 dollar and 20 cents, and 4 comma 1 dollar and 60 cents. The relationship between the number of shells and their cost is Choose... .