6. ………….is essential to life in plants and animals.
A. Nitrogen B. The nitrogen
C. That nitrogen D. It is nitrogen
7. North America has ………….. of 9,635,000 square miles.
A. a area B. the area
C. areas D. an area
8. Nomads are people who wander from place to place and have …………….. permanent home.
A. not B. none
C. nothing D. no