It’s said that this kind of plant only gives off this strange perfume on ________ conditions which the scientists are trying to figure out now.

His parents thought he was working very hard in the room, _____in fact, he was chatting with his friends on the Internet.

She says that she’ll never forget the time ________ she works as a secretary in our company.

Thelack of eco-friendly habits among the public is thought to be a major _____ofglobal climate change.

The workers insist that the company should take measuresto improve their working condition