An $\textit{in vitro}$ translation system can synthesize peptides m all three reading frames of the RNA template. When $\text{5’ - UCUCUCUC --- (UC)_n---UCUCUCUC - 3’}$ was used as the template in this $\textit{in vitro}$ translation system, the synthesized peptides contained $50\%$ each of serine and leucine. When $\text{5' - CCUCCUCCU---(C CU)}_n---\text{CCU CCU} - 3$' was used as the template, the synthesized peptides contained $33.3 \%$ each of serine. leucine, and proline. Deduce the codon for proline.
- $UCU$
- $CUC$
- $CCU$
- $UCC$