Question

nasa, which continues to research the effects of space travel on humans, has a lot more work to do to make it safe for humans to travel to mars.
reader
before ▼ can ▼ humans to ▼, they need to ▼ the effects of traveling to space.
answer bank
mars
learn more about
nasa
safely send

Explanation:

The sentence structure is "Before [subject] can [action] humans to [destination], they need to [action related to research] the effects of traveling to space."

• "NASA" is the subject (organization doing the action).
• "safely send" is the action related to sending humans.
• "Mars" is the destination (where humans are sent).
• "learn more about" is the action related to researching the effects.

Answer:

Before NASA can safely send humans to Mars, they need to learn more about the effects of traveling to space.