The ro– group is the nucleophile. think about which group in the alkyl halide (iodoethane) is the leaving group. keep in mind that the leaving group may be one atom or a group of atoms.

When Nucleophile (Alkoxide) reacts with Alkyl Halide (Substrate), Ether (product) is formed with the elimination of Leaving group (Iodide in this case).

This is SN² a nucleophilic substitution reaction which takes place in primary alkyl halides. Reaction is given as, 

Answer is to avoid experimental error