Emma is curious to see how fertilizer makes a plant grow larger. after doing some research, she believes that the chemicals within the fertilizer will make a plant increase in size. she also believes that plants that do not receive fertilizer will be smaller in size. emma plans to put the same type of plant seed into 6 pots containing soil and fertilizer and monitor how the plants grow. she runs out of fertilizer, though, and decides to put sand into 3 of the pots instead of fertilizer and soil. after the experiment finishes, emma observes that the plants that had fertilizer grew much larger than the plants with sand. she finally concludes that fertilizer greatly increases the size of a plant. why is her conclusion invalid?