Research for a Universal COVID-19 Vaccine Is Primarily Focused on

Research for a universal COVID-19 vaccine is primarily focused on

A) nucleocapsid proteins, as they do not mutate as rapidly as S proteins, and they tend to be similar between different coronaviruses.
B) the S protein because it mutates rapidly, which increases the longevity of the vaccine.
C) viral tails because they exist in all coronaviruses and do not mutate often.
D) the E protein because it tends to vary between different coronavirus strains.