3. For the geometric sequence $\left\{a_{n}\right\}$ with all terms being real numbers, the sum of the first $n$ terms is denoted as $S_{n}$. If $S_{10}=10, S_{30}=70$, then $S_{40}$ equals
A. 150
B. -200
C. 150 or -200
D. 400 or -50
Let $\left\{a_{n}\right\}$ be a geometric sequence with common ratio $q$, then $S_{10}, S_{20}-S_{10}, S_{30}-S_{20}, S_{40}-S_{30}$ form a geometric sequence with common ratio $q^{10}>0$. Therefore, $10+10 q^{10}+10 q^{20}=70 \Rightarrow q^{10}=2 \Rightarrow S_{40}=10\left(1+2+2^{2}+2^{3}\right)$ $=150$. Hence, the answer is $A$.