When we apply set operators (UNION, INTERSECT, EXCEPT) to two relations R and S, which conditions on R and S must be satisfied?
a. R and S must have schemas with identical sets of attributes and the types (domains) for each attributes must be the same in R and S
b. Before we compute the set-theoretic union, intersection, or difference of sets of tuples, the columns of R and S must be ordered so that the order of attributes is the same for both relations
c. All of the others
Answer: C