There is a logical operation in computer language. If two numbers are 0 in the same digit, the sum is 0; one is 0, one is 1 or both are 1, the sum is 1. So, what? ['If the sum is 1, then both numbers must be 1.', 'If the sum is 0, then both numbers must be 0.', 'If the sum is 0, one of the two numbers may be 1.', 'If the sum is 1, at least one of the two numbers is 0.']