Which of These Statements Is Best

Which of these statements is best?

A) Because slack variables yield no profit, they are not included in the objective function when creating the initial simplex tableau.
B) For a maximization problem, the Z_j values in the body of the simplex table represent the gross profit given up by adding one unit of this variable into the current solution.
C) In any linear programming problem, if a variable is to enter the solution, it must have a positive coefficient in the C_j - Z_j row.
D) In a maximization problem, the C_j - Z_j row gives the net loss from introducing one unit of each variable into the solution.