45. ___
A. The extent of damage to the nervous system.
B. The presence or absence of certain nerve-cell receptors.
C. The size of nerve-cell receptors in the brain.
D. The level of danger in the mammal’s environment.
46. ___
A. To show the relationship between fearfulness and environment.
B. To give examples of animals that aren’t fearful.
C. To compare fear in mammals to fear in other animals.
D. To identify the nerves that control fear in certain animals.
47. ___
A. Why water flows from artesian springs.
B. How artesian wells are drilled.
C. Why artesian springs are important to geologic research.
D. How aquifers are formed.
48. ___
A. They pump water from the aquifer.
B. They purify the water in the aquifer.
C. They store excess water from the aquifer.
D. They trap water in the aquifer.
49. ___
A. By eroding layers of sediment above it.
By traveling through cracks in layers of rock.
C. By reversing its flow down the aquicludes.
D. By boiling up through pores in the aquifer.
微信支付 
支付宝支付