单项选择题
A．is insignificant B．seems overestimated C．is enormous D．remains unknown
A．the Apollo mission was very successful B．protection from space radiation is not an easy job C．astronauts will have deformed children or grandchildren D．radiation is not a threat to well-protected space explorers
A．Uncontrolled development. B．A warming atmosphere at a global level. C．Sunlight reaches the forest floor, dries plants and lights fires. D．No limits on logging and bad fire preventing efforts.