B. because it is dependent on something to manipulate (change) it, that is why it is called a dependent variable :)
In science a hypothesis is useful only if...
1: it could be tested
2: their are two separate group
3: it's supported by evidence and facts
Answer:
The growth of the plants is the dependent variable, the stuff added to each plant is the independent variable, the plants that don't receive treatment/other stuff are control variables.