Write a C program substring.c. Write a function called substring to extract a portion of a character string. The function should be called as follows: substring (source, start, count, result); where source is the character string from which you are extracting the substring, start is an index number into source indicating the first character of the substring, count is the number of characters to be extracted from the source string, and result is an array of characters that is to contain the extracted substring. For example, the call substring ("character", 4, 3, result); extracts the substring "act" (three characters starting with character number 4) from the string "character" and places the result in result. Be certain the function inserts a null character at the end of the substring in the result array. Also, have the function check that the requested number of characters does, in fact, exist in the string. If this is not the case, have the function end the substring when it reaches the end of the source string. So, for example, a call such as substring ("two words", 4, 20, result); should just place the string words inside the result array, even though the call requested 20 characters