A farm has two tanks, tank A and tank B, which are being filled with water at constant rates. Tank B already has 150 gallons of water by the time water starts filling in tank A. Consider the graph below which shows the relationship between the amounts of water in tank A and tank B in gallons, and the time elapsed, in hours. Which of the lines shows a proportional relationship, and what is the constant of proportionality? A. Tank B's line shows a proportional relationship, and the constant of proportionality is 150. B. Tank A's line shows a proportional relationship, and the constant of proportionality is 0.02. C. Tank A's line shows a proportional relationship, and the constant of proportionality is 50. D. Tank B's line shows a proportional relationship, and the constant of proportionality is 225.