209. How many of the following statements are incorrect?
In the structure of polymers, a carbon atom can have two double bonds.
For participation in addition reactions, the presence of a double bond in the four-membered ring structure is necessary.
The structural units of fiber polymers are connected to each other by carbon--carbon bonds.
In condensation polymerization, based on the number of monomers used, a specific product is formed.
(1) four (2) three (3) two (4) one
\textbf{209.} How many of the following statements are \underline{incorrect}?
\begin{itemize}
\item In the structure of polymers, a carbon atom can have two double bonds.
\item For participation in addition reactions, the presence of a double bond in the four-membered ring structure is necessary.
\item The structural units of fiber polymers are connected to each other by carbon--carbon bonds.
\item In condensation polymerization, based on the number of monomers used, a specific product is formed.
\end{itemize}
(1) four \quad (2) three \quad (3) two \quad (4) one