单项选择题
A．They are told in the slaves' own voices. B．People travel around the world to hear them. C．Colored people were sold. D．They happened in the slavery time.
A．ensure the moon traveling and the moon base construction B．speed up the moon base construction and other activities C．guarantee the quality of the researchers' scientific study D．set up new training laboratories for future missions to Mars
A．Entering the orbit and landing on the moon. B．Landing and walking on the moon surface. C．Exploration and researches into the moon composition. D．Exploration and construction of a moon base.