Question

Which word in the sentence is the action verb? Comets travel around the sun. A. travel B. sun C. around D. Comets

Answer 1

Travel.
A verb is an action word. Something is doing something. Travel is the only word here which describes an action.

Answer 2

The answer to your question is A - Travel.
Travel in this sentence is an action verb. 
An action verb is where it describes doing a particular thing.

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