$2\mathrm{.}5$ Question $2.($e$)$

Suppose $$ is an input to a decision problem A and $$ is an input to decision problem $B$

Suppose also that a reduction algorithm, $R,$ that runs in polynomial time, transforms $$ into $$ and that $A()=B().$

Consider the case where $B$ is known to be complexity class $P.$ Explain why $A$ is also in complexity class $P$