\item How many nodes does a \textit{Reduced Ordered Binary Decision Diagrams (ROBDDs)} for a Boolean formula with $n$ variables have, in worst-case?

\begin{itemize}
  \item[$\square$] $2n$
  \item[$\square$] $\mathcal{O}(n^2)$
  \item[$\square$] $\mathcal{O}(2^n)$
  \item[$\square$] $2^{n+1}-1$
  \item[$\square$] $n^2$
  \item[$\square$] infinitely many
\end{itemize}