单项选择题
A．have never existed B．have all died out C．may exist in the future D．are in existence
A．is very confused about the origin of the Earth B．might possibly reach life outside the C．believes it is possible to step off the Earth D．is historically incapable of exploring the universe
A．forcefully and complexly B．elaborately and literally C．simply and humorously D．snobbishly and cleverly