6. Given the arithmetic sequence $\left\{a_{n}\right\}$, and $S_{5}=28, S_{10}=36$, then $S_{15}$ equals ( ).
A. 80
B. 40
C. 24
D. -48
6. C. It is easy to know that $S_{5}, S_{10}-S_{5}, S_{15}-S_{10}$ form an arithmetic sequence, i.e.,
$$
2\left(S_{10}-S_{5}\right)=S_{5}+\left(S_{15}-S_{10}\right) \text {, }
$$

i.e., $S_{15}=3\left(S_{10}-S_{5}\right)=3(36-28)=24$.