(a) To log on to a certain computer account, the user must type in a 4-letter password. In such a password, no letter may be repeated, and only the lower case of a letter may be used. How many such 4-letter passwords are possible? (There are 26 letters in the alphabet. ) (b) There are 15 appetizers available at a restaurant. From these, Jenny is to choose 13 for her party. How many groups of 13 appetizers are possible