In an experiment for science class, six pepper plants grown in individual containers are separated into two groups. one group is placed in a room that gets sunlight, and the other group is placed in a dark room. all of the plants receive the same amount of water daily. after three weeks, the plants are observed.

why were all the plants in the experiment pepper plants?

This relates to the problem question of the experiment. I believed the problem question should have been something like:

What is the effect of sunlight over plant growth.

When an experiment is design, the variables should be maintain constant. So if you want to test the effect of sunlight o the effect of a certain chemical this should be applied to the same type of plants in order to be able to compare and also to be sure the different data is in fact caused by what is being tested and not caused by the organism.  

This relates to the problem question of the experiment. I believed the problem question should have been something like:

What is the effect of sunlight over plant growth.

When an experiment is design, the variables should be maintain constant. So if you want to test the effect of sunlight o the effect of a certain chemical this should be applied to the same type of plants in order to be able to compare and also to be sure the different data is in fact caused by what is being tested and not caused by the organism.  

yes? sounds right, but you didn’t ask a question.