CN-LEAGUE 1998 Problem 3

For geometric series $(a_n)$ with all items real, if $S_{10}=10,S_{30}=70$ , then $S_{40}=$ $\text{(A)}150 \text{(B)}-200 \text{(C)}150\text{ or }-200 \text{(D)}-50\text{ or }400$ Note: $S_n=\sum_{i=1}^{n}a_i$ .