There are four balls of the same size in a box. The balls are numbered 1, 2, 3, and 4. Connie randomly picks up one ball, puts it back, and then randomly picks up one ball from the box. Let X denote the sum of the numbers on the two balls she picked up. a. Find the probability distribution and events corresponding to the values of X. b. Obtain the cumulative distribution function of X. c. Find P(3 X < 7).