The number of people arriving for treatment at an emergency room can be modeled by a poisson process with a rate parameter of four per hour. (a) what is the probability that exactly three arrivals occur during a particular hour? (round your answer to three decimal places.) (b) what is the probability that at least three people arrive during a particular hour? (round your answer to three decimal places.) (c) how many people do you expect to arrive during a 30-min period? people