Question: Let S be a set with n elements and let a and b be distinct elements of S. How many relations R are there on S such that
a) (a, b) ∈ R?
b) (a, b) ∈ R?
c) no ordered pair in R has a as its first element or b as its second element?
d) at least one ordered pair in R either has a as its first element or has b as its second element?