a. true b. false
a. about the same as the population of the United States b. about twice the population of the United States c. about four times the population of the United States
a. 20% b. 50% c. 70% d. 80%
a. Thailand b. Indonesia c. Philippines d. South Korea
a. three minutes b. thirty minutes c. one hour d. two hours
a. The air cannot move quickly enough to prevent a vaccuum from forming behind the moving end of the wipe. Air pressure causes air to move rapidly into that vaccuum. The rapid movement of air causes the sound. b. The entire length of the wipe suddenly is stretched tight. That causes the whole length of wipe to move as a unit and at a very fast speed. That moves a large quantity of air quickly. That movement of air is the noise that is heard when the wipe cracks. c. As the wipe is straightened and then relaxed, the sudden change of tension on the wipe causes the diameter of the wipe to first thin and then get larger. As it expands very rapidly air is pushed out suddenly in all directions causing the loud crack. d. The crack of the whip is caused by an extreme compression of the air in front of the whip which results because the end of the whip is traveling faster than the speed of sound and so air is not pushed out of the way in a normal manner. e. a and d
a. gone up b. staid the same c. gone down
a. 1 mile b. 10 miles c. 100 miles d. 1000 miles
a. the height of trees b. a person's height c. the height of the lawn grass d. both b and c