The slope of a speed-time linear graph is acceleration direction speed velocity

The answer is acceleration because the slope is how the sharp or steep the speed is increased. In a velocity versus time graph, any small part of the curve can be written as V = m T + b; where the slope of the curve is equals to dV/dT. b = U is the primary speed when time is equals to 0. In physics, the change in velocity over an interval of time is defined as acceleration.

The independent variable would be the fertilizer as it is what the dependent variable (the growth of grass) depends on.