You need to contact a client on the phone. You have estimated that you have a 70% chance of contacting the client on the first call. If you don't, you estimate that you have a 20% chance of contacting the client on a subsequent call in the next hour. Which of the following is true?

a. The probability that you will need at most two calls to contact the client is 0.90.
b. The probability that you will fail on call 1 but will succeed on call 2 is 0.06.
c. The probability that you will fail on call 1 but will succeed on call 2 is 0.50.
d. The probability that you will need at most two calls to contact the client is 0.80.