7. Fredrick wanted to find out what soil works best for growing roses. He grew them in potting soil, clay, sand, and soil he found outside his yard.
After one week, he measured their height
and how many leaves they had.
a. Independent Variable (IV): What soil works best
b. Dependent Variable (DV):
C. Control:
d. Constants:

Dependent variable are those variables that changes with the change in variables that affects it. (IG - leo_muan)

Explanation:

Dependent variable responds or gets affected to change. They are not constant. It can be possible that dependent variable is relying on independent variable as independent variable does not change with change of any of the factor in an experiment.  

Sometimes, mentioning not the units leads to insufficient data of the variables. A constant is something whose value cannot be changed and is used for comparison purpose. According to the question, Rose is constant, height is dependent variable , different types of soil are independent variable.