A homogenous reaction $(R{\rightarrow}P)$ occurs in a batch reactor. The conversion of the reactant $R$ is $67\%$ after $10$ minutes and $80\%$ after $20$ minutes. The rate equation for this reaction is

1. $-r_{R}=k$
2. $-r_{R}=kC_{R}^{2}$
3. $-r_{R}=kC_{R}^{3}$
4. $-r_{R}=kC_{R}^{0.5}$