air becomes ____and ____(越来越薄)as the plane ____(climb)in the sky

air becomes ____and ____(越来越薄)as the plane ____(climb)in the sky
air becomes ____and ____(越来越薄)as the plane ____(climb)in the sky

air becomes ____and ____(越来越薄)as the plane ____(climb)in the sky
air becomes 【thinner 】and 【thinner 】as the plane 【climbs】in the sky