America will never again have as a nation the spirit of adventure as it ______ before the West was settled.

Over the weekend, my friend asked me for help. Her temperature was 39.7C and she was ______ a fever.

. He apologized for _____. 

---where have you been?---I___in the heavy traffic, or I would have been here earlier.

Low-carbon lifestyle is of great benefit to improve the world environment, but the problem is difficult to _________．