Why do you think that some vaccines, such as the flu vaccine, need to be given every year?

Because flu viruses evolve so quickly, last year's vaccine may not protect you from this year's viruses. New flu vaccines are released every year to keep up with rapidly adapting flu viruses.

Because the virus/sickness would evolve and that vaccine might not protect you.

answer: c. two haploid cells

explanation:

the backbone of the dna and rna compound is sugar phosphate.