need help in the following select the correct answer

What is the difference between balanced and unbalanced panel? O a. Balanced panels have an equal number of observations above and below the mean of the dependent variable, unbalanced panels are skewed. O b. Unbalanced panels have some observations missing, balanced do not. O c. Balanced panels are demographically representative of the population being studied, unbalanced are not. O d. A balanced panel has T = N, an unbalanced panel has N>T or N&It; T