Question

3. After studying about recycling, members of John’s biology class investigated the effects of various recycled products on plant growth. John’s lab group compared the effect of different aged grass compost on bean plants. Because decomposition is necessary for the release of nutrients, the group hypothesized that older grass compost would produce taller bean plants. Three flats of bean plants (25 per flat) were grown for 5 days. The plants were then fertilized as follows: flat A = 450. g of 3 month old compost; flat B = 450. g of 6 month old compost; flat C = 0 g of compost. The plants received the same amount of sunlight and water each day. At the end of 30 days, the group recorded the height of the plants in cm.
Hypothesis:
Independent Variable:
Dependent Variable:
Constant (at least one):
Control:
Number of groups:
Number of trials per group:

Answer 1

The hypothesis may be,
"If the compost is older, then the height of the bean plants will be taller."
Because it relates the independent and dependent variables

The independent variable is the age of compost
Because it is the variable being changed
 
The dependent variable is the plant height of the bean plant 
Because it is the variable on which the effect of the independent variable is being investigated

The constant are the sunlight and water provided.
Because they are the same for each group

Flat C is the control group
Because it is given no compost

There are 3 groups and each group has 25 trials.