单项选择题
A．is insignificant B．is enormous C．seems overestimated D．remains unknown
A．the Apollo mission was very successful B．protection from space radiation is no easy job C．astronauts will have deformed children of grandchildren D．radiation is not a threat to well protection space explorers
A．it protects him against the harmful rays from space B．it provides sufficient light for plant growth C．it supplies the heat necessary for human survival D．it screens off the falling meteors