Which of the following random variables is geometric? (2 points) a The number of 4s in a row of 50 random digits. b The number of digits in a randomly selected row until a 4 is found. c The number of tails when a coin is tossed 30 times. d The number of spade cards obtained in a seven-card deal-out of a shuffled deck of 52 cards. e The number of 4s when rolling a die 5 times.