Example 3 (1998 National High School Competition Question) For the geometric sequence $\left\{a_{n}\right\}$ with real number terms, the sum of the first $n$ terms is $S_{n}$. If $S_{10}=10, S_{30}=70$, then $S_{40}$ equals ( ).
A. 150
B. -200
C. 150 or -200
D. 400 or -50