Which of the following random variables is geometric?

a. the number of 5s when rolling a die 35 times
b. the number of spades dealt from a shuffled deck of 52 cards in a seven card hand
c. the number of heads when a coin is tossed 10 times
d. the number of digits in a randomly selected row until a 9 is found
e. the number of 8s in a row of 20 random digits