National Party Conventions Occur How Frequently,and What Purpose Do They

National party conventions occur how frequently,and what purpose do they serve?

A) every two years; to choose the party platform
B) every two years; to select the party's presidential and vice presidential nominees
C) every four years; to select the party's presidential and vice presidential nominees
D) prior to every state and national election; to choose the party platform
E) after every state and national election; to decide party policy