\textbf{81--} Which case is correct?
\begin{enumerate}
\item Gases, unlike solids and liquids, have no definite volume or shape.
\item By increasing the pressure on a gas sample, the volume of its molecules decreases.
\item The distance between molecules of a gas sample is a function of the pressure exerted on it.
\item At constant temperature and pressure, the volume of one gram of gas CO equals the volume of one gram of gas $CO_2$.
\end{enumerate}
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\textit{Calculation Space}
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