21) B

1 – Correct. Amino acids are encoded by a group of 3 bases. 166 x 3 = 498. So a gene consisting of 500 base pairs will have a maximum of 166 amino acids.

2 – Wrong. Remember, red blood cells have no nucleus, hence no nuclear chromosomes.

3 – Wrong. Adenine binds with thymine or uracil, not with guanine. So it is not necessary that the number of adenine bases in an allele must be the same as the number of guanine bases.

So, B is the answer.