how can I extend the width of item?

Question

Please see the picture first. I want to keep the first width of item rectangle, meanwhile extend the second item rectangle, make it longer.

I mean that I'd like to let the leftmargin & rightmargin much more smaller, so that more contents can be displayed on one line. How can I do that without define a new environment?

I see many related pages, but I cannot solve the problem. Be grateful with any help!

\begin{frame}    
\begin{columns}[t]%
\begin{column}[t]{0.45\textwidth}
\begin{itemize}
     \item A.~~4
     \item C.~~2
  \end{itemize}
    \end{column}
 \begin{column}[t]{0.45\textwidth}%
     \begin{itemize}
    \item B.~~3
    \item D.~~1
  \end{itemize}
\end{column}
\end{columns}

\begin{itemize}[leftmargin=0cm]
\item $y=x^3$: $f(-x)=(-x)^3=-x^3, -f(x)=-x^3$. $\Rightarrow$ $f(-x)\neq -f(x)$\\
\item $y=2^x$: $f(-x)=2^(-x)=2^{-x},-f(x)=-2^x$.$\Rightarrow$ $f(-x)\neq -f(x)$\\
\end{itemize}
\end{frame}

Answer 1

without defining a new environment/template, you could simply enclose the itemize env into a group with a modified leftmargini, like this:

\documentclass{beamer}

\begin{document} 
\begin{frame}    
\begin{columns}[t]%
\begin{column}[t]{0.45\textwidth}
\begin{itemize}
     \item A.~~4
     \item C.~~2
  \end{itemize}
    \end{column}
 \begin{column}[t]{0.45\textwidth}%
     \begin{itemize}
    \item B.~~3
    \item D.~~1
  \end{itemize}
\end{column}
\end{columns}

{                                     %new code: create a group
\setlength{\leftmargini}{0cm}         %new code: set margin length to 0 
\begin{itemize}
\item $y=x^3$: $f(-x)=(-x)^3=-x^3, -f(x)=-x^3$. $\Rightarrow$ $f(-x)\neq -f(x)$\\
\item $y=2^x$: $f(-x)=2^(-x)=2^{-x},-f(x)=-2^x$.$\Rightarrow$ $f(-x)\neq -f(x)$\\
\end{itemize}
}                                     %new code: close the group

\end{frame}
\end{document}