A power system with two generators is shown in the figure below. The system (generators, buses and transmission lines) is protected by six overcurrent relays $R_{1}$ to $R_{6}$. Assuming a mix of directional and nondirectional relays at appropriate locations, the remote backup relays for $R_{4}$ are

1. $R_{1}, R_{2}$
2. $R_{2}, R_{6}$
3. $R_{2}, R_{5}$
4. $R_{1}, R_{6}$

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