Complex sentence: You can derive this relation directly-A depends on B and C, because you can add two variables only if and only if you know the values of both variables, thus, B and C must be calculated before A can be calculated.
Split into simpler sentences: You can derive this relation directly-A depends on B and C, because you can add two variables if and only if you know the values of both variables. Thus, B and C must be calculated before A can be calculated.