Parameter: An array of strings 
• Return Value: A list of three strings that occur most frequently in the parameter array. 
Exercise 9.4
• Parameter: An array of numbers (pass by value) and two arrays (pass by reference)
• Return Value: None.
• Result: The first pass-by-reference parameter must have the values of the given array that are greater than zero; the second must have the values that are less than zero.
Exercise 9.6
• Parameter: A file variable of a file of text, where the words are separated by spaces or colons. 
• Return Value: The word that appears most often in the file. 

Exercise 9.7
• Parameter: A string containing words that are delimited on the left with spaces and on the right with spaces, commas, periods, or problem marks.
• Return Value: The three most common words in the string that have more than three letters.