What Do You Think Might Be a Side-Effect of or a Cause

What do you think might be a side-effect of or a cause of nations with very high voter turnout having that level of turnout?

A) They may have a kind of political fever in which partisan politics has become too intense.
B) They likely play host to elections in which indistinct personalities and a relatively unified electorate bring out more voters.
C) They probably don't offer automatic voter registration.
D) They usually have mandatory voting.