1. [8 marks] Sometimes it is useful to be able to tell if a credit card number is valid or not. For each of the following credit card numbers, perform the Luhn check algorithm to get the sum (see the clip on Blackboard), indicate if the credit card number is valid and if it is not, what would the valid credit card number be? Please show your work. a. 4520 8800 5785 2358 b. 5420 3835 6729 8133 c. 6532 1098 7653 2108 d. 5084 2784 4287 2276