1. Emily filled an empty horse trough with water. The water level, in centimeters, is proportional to time elapsed, in minutes, since she began filling the trough. The graph shows this relationship. Water Trough Level у 35 30 y = 3.5x 25 (8, 28) (6, 21) Water level (cm) 20 h 15 + 10 + 5 (0,0) 0 2 3 4 5 8 9 10 (1, 3.5) 6 7 Time (min) (a) Identify the constant of proportionality: (b) Identify the unit rate: PER (c) Explain what (8, 28) means in terms of the graph. Use a complete sentence! (d) What would the water level be after 20 minutes (hint: use a proportion!)?