Answer:
B
Explanation:
Mendel needed to control the plants' pollination, because he wanted to selectively breed the plants and observe the results.
Why not A: He began with purebreeding plants, so he could trace alleles exactly.
Why not C: He observed so many generations! It's important in an experiment to have as large a sample size as you can without being too large.
Why not D: Part of the reason he picked pea plants is because they reproduce quickly, so he could measure efficiently.
