What is the two$-$party system?

a$)$ a system in which political parties tend to form factions within themselves, that is to say, a single political party forming a liberal and a conservative wing

b$)$ a system, laid out in the Constitution, that calls for only two major parties to compete in most elections

c$)$ a system in which political parties act at two levels; a local level more responsive to members, and a national level more responsive to countrywide interests and groups

d$)$ a system in which only two parties have a realistic opportunity to compete effectively for control