What Does Turnover Mean in Regard to the U

What does turnover mean in regard to the U.S. Congress?

A) Turnover does not apply to the Congress, it applies to the presidency
B) Turnover as it applies to the Congress means the number of seats in the Congress which change hands from one party to another
C) Turnover as it applies to the Congress means the number of seats in the Congress which change hands from one person to another
D) Turnover as it applies to the Congress means the number of seats in the Congress which change hands from some demographic indicator like race, religion, gender, etc., to another