Fill in the gaps with the correct word derived from the word in brackets

1. Big Ben is a popular tourist [ ] in London.
2. Establishing a world clock was a very [ ] problem.
3. The scientists gathered in the [ ] to study the stars.
4. A(n) [ ] is an excellent place to learn about astronomy.
5. The prime meridian is a(n) [ ] line across the world.
6. You are not allowed to [ ] touch the exhibits in the museum.