6. Given the sequence $\left\{a_{n}\right\}$ is an arithmetic sequence, and $S_{5}=$ $28, S_{10}=36$. Then $S_{15}$ equals ( ).
(A) 80
(B) 40
(C) 24
(D) -48
6.C.

It is known that $S_{5}, S_{10}-S_{5}, S_{15}-S_{10}$ form an arithmetic sequence. It is easy to get $S_{15}=24$.