The Notation for Panel Data Is (Xit,Yit),i = 1

The notation for panel data is (Xit,Yit) ,i = 1,... ,n and t = 1,... ,T because

A) we take into account that the entities included in the panel change over time and are replaced by others.
B) the X's represent the observed effects and the Y the omitted fixed effects.
C) there are n entities and T time periods.
D) n has to be larger than T for the OLS estimator to exist.