Let S = {0, 1} and consider the partial order relation R defined on S ✕ S as follows: For all ordered pairs (a, b) and (c, d) in S ✕ S, (a, b) R (c, d) ⇔ a ≤ c and b ≤ d, where ≤ denotes the usual "less than or equal to" relation for real numbers. Find a chain of length 2 for R.