Let X = RtU {0} and d: X x X R be a function defined by d(x, y) = max{r, y} Then, (X, d) is a metric space. Let A = (1, 2]U {3}. Which of the following sets A, A' and A, respectively? a) O (1, 2] [1, 2] [1, 2] U {3} (1, 2] U {3} [1, 2)] U {3} b) (1, 2] [1, 2] c) (1, 2] U {3} d) (1, 2) [1, 2] e) . None of them