Io is locked in a gravitational resonance orbit with which two other large Jovian moons?

Answer

Europa and Ganymede

Io is caught in a gravitational tug-of-war, locked in a resonance orbit specifically with Jupiter's moons Europa and Ganymede, which generates the tidal flexing.

Io is locked in a gravitational resonance orbit with which two other large Jovian moons?
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