Modern communications satellites use a variety of orbits, including ( ). A: geostationary orbits B: Molniya orbits C: elliptical orbits D: low (Polar and non-polar) earth orbits

The truth ________ the moon orbits the sun is doubted by some scientists.

Most asteroids, which are made of rock, ＿themselves＿ a belt between the orbits of Mars and Jupiter.

The space station is made of parts that were assembled in space by astronauts and ________ Earth at an average altitude of approximately 250 miles. A: programs B: spins C: travels D: orbits

The United States (has sent) several (spacecrafts) into orbits (around the earth) and has collected a lot of (information)（） A: has sent B: spacecrafts C: around the earth D: information

Moisture in food can be divided into "bound water" and "free water" according to different properties. Which of the following is not bound water? A: The first layer of water molecules adsorbed to glucose molecules B: Water molecules with unpaired electrons entering the outer empty orbits of sodium ions C: Water molecules adsorbed in capillaries in food tissue D: Water molecules bound to the negative ion of glutamic acid on the protein peptide chain