A) Yes, the graph represents a proportional relationship because the rate at which the plane's altitude increases is directly proportional to the amount of time that has elapsed since takeoff.
B) The unit rate of change is the number of feet per minute that increases over time, which can be calculated by finding the slope of the line. To do this, we choose two arbitrary points on the line and calculate the change in altitude divided by the change in time for each point. Therefore, if we choose the points (x₁, y₁) = (0, 0) and (x₂, y₂) = (2, 12000), then the unit rate of change is (12000 - 0) / (2 - 0).
The unit rate of change is 6000 feet per minute, calculated by (12000 - 0) / (2 - 0).
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