Answer
a) $\Sigma a_{n}$ is divergent.
b) You cannot say anything about $\Sigma a_{n}$
Work Step by Step
a) Given $a_{n} > b_{n}$ for all $n$ and $\Sigma b_{n}$ is divergent, since $a_{n} \geq b_{n}$ is true for all $n$, $\Sigma a_{n}$ is divergent by the Comparison Test.
b) To use the Comparison Test, $a_{n} \geq b_{n}$ for all $n$ when $\Sigma b_{n}$ is divergent. If $a_{n} < b_{n}$, we cannot use the comparison test, and therefore, we cannot say anything about $\Sigma a_{n}$.