Use C language

3. Complete Project 7 in Chapter 9 on page 569 of Hanly & Koffman Problem Solving and Program Design in C. Write a recursive function that displays all the binary (base 2) numbers represented by a string of xs, 0s, and 1s. The xs represent digits that can be either 0 or 1. For example, the string 1x0x represents the numbers 1000, 1001, 1100, 1101. The string xx1 represents 001, 011, 101, 111. Hint: Write a helper function replace_first x() that builds two strings based on its input argument. In one, first x is replaced by a 0, and in the other by a 1. The set function is_element() may be useful too