The word astronauts in the sentence is functioning as the direct object.
Direct objects are often a noun phrase that refers to a person or thing that <u>receives</u> the action of the verb. In order to spot a direct object within a sentence, remember to ask yourself “who” or “what” is receiving the action. The answer in this case would be: <em>"the austronauts". </em>
A, C, and D all have commas where they are NOT necessary, so they are sentences that are considered NOT to have proper punctuation. Thus, the answer is B.
