SAT Question 96c720af: Form Structure And Sense answer and explanation

English - Form Structure And Sense - Verb Tense Consistency
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Atoms in a synchrotron, a type of circular particle accelerator, travel faster and faster until they ______ a desired energy level, at which point they are diverted to collide with a target, smashing the atoms.
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Difficulty:Easy
Which choice completes the text so that it conforms to the conventions of Standard English?