CN-LEAGUE 1998

If $a>1,b>1,\lg(a+b)=\lg a+\lg b$ , then the value of $\lg(a-1)+\lg(b-1)$ is $\text{(A)}\lg2 \text{(B)}1 \text{(C)}0 \text{(D)}$ not sure

Nonempty set $A=\{x|2a+1\leq x\leq 3a-5\},B=\{x|3\leq x\leq22\}$ , then the range value of $a$ such that $A\subseteq A\cap B$ is $\text{(A)}\{a|1\leq a \leq9\} \text{(B)}\{a|6\leq a \leq9\} \text{(C)}\{a|a \leq9\} \text{(D)}\varnothing$

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: $S_n=\sum_{i=1}^{n}a_i$ .