Question

how did they know that it was the amount of water that made the plant grow bigger? actually, they could not be sure because it was the dependent variable because there was only one independent variable and everything else was kept the same that was their hypothesis

Explanation:

In a scientific experiment, to determine the effect of a variable (here, amount of water) on a result (plant growth), one - variable - at - a - time testing is done. If only the amount of water is changed while other factors like sunlight, soil type, temperature are kept constant, it can be reasonably concluded that the water amount is influencing plant growth.

Answer:

Because there was only one independent variable and everything else was kept the same