1. Anything that drops falls towards the centre of the earth because of the pull of g______.
2. The company’s head office is in Beijing, but it has b______ all over the country.
3. In t______ it is possible; in practice it is not.
4. It is good to be ______ (好奇的) about the world around you.
5. The river is the ______ (边界线) between the two countries.
