21) D

1 – Since 1 of the phenotypes was not seen, it means that the offspring with both RECESSIVE alleles did not survive.

2 – For any scientific research, a large sample size is very important as it reduces the effect of chance. Therefore, this statement is correct. A small sample size could be the reason why a different ratio was seen.

3 – Chance could also be a reason.

Hence, D is the answer.