Which expression represents the number of cells after half an hour?
A $2^{10}$
B $2^{15}$
C $2^{20}$
D $2^{30}$
$$\begin{array}{|c|c|}
\hline \begin{array}{c}
\text { Time } \\
\text { (min) }
\end{array} & \begin{array}{c}
\text { Number of } \\
\text { Bacteria }
\end{array} \\
\hline 0 & 2^{0} \\
\hline 3 & 2^{1} \\
\hline 6 & 2^{2} \\
\hline 9 & 2^{3} \\
\hline 12 & 2^{4} \\
\hline
\end{array}$$