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Help the rookie astronaut finish writing her notes on the space station's safety manual.
The airlock doors _________________________ completely before you remove your space helmet. Otherwise, you __________________________ into the vacuum of space, which ruins everyone's afternoon!
The airlock doors must be sealed completely before you remove your space helmet. Otherwise, you could be sucked into the vacuum of space, which ruins everyone's afternoon!
The passive voice is used because the subject is receiving the action.
The doors don't seal themselves; they must be sealed (modal + be + past participle).
You don't suck anything; you could be sucked out by the vacuum!