All the astronauts ... the earth in space capsules are weightless.

A. orbit  B. orbiting  C. orbited  D. are orbiting

Our home planet, Earth, resides in a glittering city of stars ... the Milky Way galaxy.

A. call  B. calling  C. to call  D. called

Hailey ... that Mercury was so far away that its parallax angle would be very difficult to determine.

A. realized  B. knew  C. learned  D. promised

Europe's biggest ISS project is the Columbus science laboratory ... astronauts can carry out scientific experiments in weightless conditions.

A. which  B. where  C. when  D. there

To walk on the moon, the astronauts had to carry a suitcase ... contained oxygen.

A. which  B. it  C. having  D. where