1. Glaciers, which are masses of ice that flow slowly over land, form in the cold polar regions and in high mountains.
2. A rebel is a person who resists, who fights against authority.
3. Petroleum, which some people refer to as black gold, is one of the most valuable resources in the world today.
4. You don't need to take heavy clothes when you go to Bangkok, which has one of the highest average temperatures of any city in the world.
(though 'which' is not the best conjunction to use here)
5. A political party is an organized group of people(,-omit) who control or seek to control a government.
6. Child labor was a social problem in late eighteenth century, England, where employment in factories became virtual slavery for children.
7.We had to use a telephone, so we went to the nearest house. The woman who answered our knock listened cautiously to our request.
8.According to a newspaper article which I read, the police arrested the man(,-omit) who had robbed the First National Bank. The man who was wearing a plaid shirt and blue jeans was caught shortly after he had left the bank.
(I can see a kind of meaning to the sentence where the comma might be used, but I don't think that's the meaning that is intended, so omit.)
9. I watched a scientist conduct an experiment on bees. The research scientist, who was well-protected before she stepped into the special chamber holding the bees, was not stung. A person who was unprotected by the special clothing could have gotten/suffered 300 to 400 bee stings within a minute.
- For Teachers