Which of the following random variables is geometric?
a. the number of 3s when rolling a die 10 times
b. the number of heart cards obtained in a seven-card deal-out of a shuffled deck of 52 cards
c. the number of heads when a coin is tossed 25 times the number of 5s in a row of 50 random digits
d. the number of digits in a randomly selected row until a 4 is found