Prove that pseudorandom generator is a one way function
$$l = \log n: \Pr[B(Un+\log n)]≥2^n/2^{n+\log n} \cdot 1/p(n)$$
Since even if the input of $B$ is from the image of $G$, the probability to invert it is $1/p(n)$.
So you need to correct this case and it will help you with $l = 1$.