6. Put ten identical balls into three boxes numbered $1, 2, 3$ (all ten balls must be placed each time), with the requirement that the number of balls in each box is no less than the number of the box. Then the number of such arrangements is ( ).
(A) 9
(B) 12
(C) 15
(D) 18
6. C.

By the partition method, we get $\mathrm{C}_{6}^{2}=15$.