2 Complete the sentences with the
phrases from Ex. 1.

1 The Earth's atmosphere contains
....... gas.
2 Space is often referred to as the .......
..; the unexplored territory.
3 The...
all the planets that orbit the sun.
4 Every new step we make in
........ is made up of
advances our knowledge of the universe.
5 The.......... film is set on Mars.
6 The threat of ........ is frightening