What Is the Main Difference Between an Incumbent Race and an Open

What is the main difference between an incumbent race and an open election?

A) In an incumbent race, only those who have voted before may vote, whereas anyone may cast a vote in an open election.
B) A sitting president is pitted against a challenger in an incumbent race, while both candidates are new in an open election.
C) An open election involves voters casting a non-secret ballot, whereas the ballot is kept secret during an incumbent race.
D) An open election happens during a midterm year, whereas an incumbent race happens during a presidential election.