Which of the following is true?
a) The product of the eigenvalues of a matrix is equal to the trace of the matrix
b) The eigenvalues of a skew-symmetric matrix are real
c) A is a nonzero column matrix and B is a nonzero row matrix, then the rank of AB is one
d) A system of linear non-homogeneous equations is consistent if and only if the rank of the coefficient matrix is less than or equal to the rank of the augmented matrix