(6) Place 10 identical balls into 3 boxes numbered $1, 2, 3$ (all 10 balls must be placed each time), with the requirement that the number of balls in each box is no less than the box's number. The number of such arrangements is ( ).
(A) 9
(B) 12
(C) 15
(D) 18
(6) C Hint: By the stars and bars method, we get $\mathrm{C}_{6}^{2}=15$.