I would say (B) - because if they are all identical, they lack variation. If a new disease were to affect the identical organisms in a population, they would all be wiped out, therefore stopping natural selection from occurring.
When antigens (foreign substances that invade the body) are detected, several types of cells work together to recognize them and respond. These cells trigger the B lymphocytes to produce antibodies, which are specialized proteins that lock onto specific antigens
18: D, homologous
20: B, DNA
I can’t see 17 or 19!