Two RNAs shown below were used separately as templates in an in vitro translation system, which can generate proteins in all possible reading frames.
\[
RNA_1: 5' - (AG)_n - 3' \quad \text{and} \quad RNA_2: 5' - (AAG)_n - 3'
\]
The RNA_1 translated product contained Arg and Glu.
The RNA_2 translated product contained Arg, Glu, and Lys.
Which one of the following codons directs the incorporation of Arg?