// C# program to check if a given // credit card is valid or not. using System; class CreditCard { // Main Method public static void Main() { long number = 5196081888500645L; Console.Write(number + " is " + (isValid(number) ? "valid" : "invalid")); } // Return true if the card number is valid public static bool isValid(long number) { return (getSize(number) = 13 && getSize(number) = 16) && (prefixMatched(number, 4) || prefixMatched(number, 5) || prefixMatched(