Some equations are solved on the basis of a certain system. On the same basis, find out the correct answer from amongst the four alternatives for the unsolved equation. 5 + 3 + 7 = 573;  6 + 3 + 9 = 693;  6 + 4 + 8 = 684;  9 + 4 + 5 = ?

594 459 945 954

954