As of a certain date, roughly 50 million web domain names were registered. a) How many domain names consisting of just two letters in sequence can be formed? (lower case only) b)How many domain names are there consisting of three letters in sequence? (lower case only) c) How many domain names are there consisting of four letters in sequence if either letters or digits are permitted? (lower case only) d) As of this certain date, 97,786 of the four -character sequences using either letters or digits had not yet been claimed. If a four-character name is randomly selected, what is the probability that it is already owned? (use your answer in c above)