tests: explain with an example what is a multiple-mode defect for a software application.

If there are 4 input variables (for example, x, y, z and w), each of which could take 2 possible values ​​(x1, x2, y1, y2, z1, z2, w1, w2), how many possible double fault modes can there be? How many test entries would you select to test in pairs? Show these test entries. How many test entries would be required for a comprehensive test?