In a Little League baseball game, team A's pitcher throws a strike 50% of the time and a ball 50% of the time, successive pitches are independent of one another, and the pitcher never hits a batter. Knowing this, team B's manager has instructed the first batter not to swing at anything. Calculate the probability that
a. The batter walks on the fourth pitch
b. The batter walks on the sixth pitch (so two of the first five must be strikes), using a counting argument or constructing a tree diagram
c. The batter walks
d. The first batter up scores while no one is out (assuming that each batter pursues a no-swing strategy)