单项选择题
A. they are short of food B. there is not enough space C. it’s getting warmer in the low land D. it’s getting coldier in the mountains
A. jump as high as you can B. slide down a rope to the ground C. tie yourself to a rope and fall D. f all without a rope
A. The lifecycle of trees B. The number of trees C. The intensity of solar burning D. The quality of air.