The United States has two major parties: Democrats and Republicans.
Democrats are more liberal, which means they believe that government
should be active in supporting social and political change. Republicans are
more conservative. They believe that the government should play a smaller
role in people's lives and value established practices and tradition in politics and society. Though particular political parties have changed over the years,
there has generally been one more liberal party and one more conservative.
Today, in addition to these two large parties, there are many smaller ones,
such as the Libertarian Party, the Green Party and the Constitution Party.
These smaller parties come together when individuals have a set of
interests that are not represented by either of the two major parties.
Based on the passage, what is the purpose of political parties other than the two major ones?

A. to convince people to leave the Democratic and Republican parties

B. to make sure there is an even number of parties on any ballot

C. to encourage the parties to change over the years

D. to advocate for interests that are not represented by the major parties