Solution
= Solution
{c}
Given $\varepsilon>0$, the two assumed <limits of sequences> supply thresholds $N_1,N_2$ such that $|a_n-\ell|<\varepsilon$ for $n\geq N_1$ and $|c_n-\ell|<\varepsilon$ for $n\geq N_2$. For $n\geq\max\{N_1,N_2\}$,
$$
\ell-\varepsilon<a_n\leq b_n\leq c_n<\ell+\varepsilon.
$$
Thus $|b_n-\ell|<\varepsilon$ eventually, proving the <squeeze theorem>:
$$
\boxed{b_n\longrightarrow\ell.}
$$