3) Write the following C program: The program reads a set of strings from an input file. The program counts the number of occurrences of every letter in the file, and then creates an output file that includes each letter and their number of occurrences. Lower and upper case letters are counted as the same letter, e.g., 'a' and 'A' are counted together. If you do not know how to solve this aspect then you can count the lower and upper case letters as different, but there will be some penalty.