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In Death Valley National Park's Racetrack Playa, a flat, dry lakebed, are 162 rocks--some weighing less than a pound but others almost 700 pounds--that move periodically from place to place, seemingly of their own volition. Racetrack-like trails in the ________ mysterious migration.
Which choice completes the text so that it conforms to the conventions of Standard English?
Difficulty: Medium
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