How does having a diverse gene pool a species survive

It helps because if there are diverse genes then it is easier to survive. For instance lets say you and a whole population of 30 have gene A and only 4 of the population has gene B if a virus comes that is only effective to gene A and the whole population of 34 had gene A the population would become extinct but since there are 4 with different genes those 4 can still thrive and reproduce.

Ithink it’s 3 as the answer.