It is merely of the universe an optical illusion of the moon ... appears considerably larger than it does.

A. who  B. whose  C. whom  D. which

More than one-third of Saturn's satellites ... in the past 20 years.

A. has been discovered  B. have been discovered  C. have discovered  D. had been discovered

He has to experience ... before becoming a professional astronaut.

A. microgravity  B. microgravitation  C. macrogravity  D. macrogravitation

This is a picture of a meteorite I took at the ... museum.

A. astronomic  B. astronomy  C. astronautic  D. astronaut

He has to experience ... before becoming a professional astronaut.

A. astronomy  B. astronomical  C. astronomer  D. astronaut