Mass extinctions have occurred ____ in the last ____.?
A) ?five times; 4.5 million years
B) ?six times; 4.5 billion years
C) ?only once; 66.5 million years
D) ?five times; 450 million years
D
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Mantle convection cells are fueled with heat from the core
a. True b. False
Which of the following is true of ethylene?
A) It is not a very important industrial chemical. B) It is present in large amounts in natural gas. C) Its most important use is as a fuel. D) The vast majority of ethylene is produced by thermal cracking.
Mayans adapted to the decline of soil fertility by what type of agricultural practice?
A) intensive subsistence B) swidden C) nomadism D) fallow farming
Desert areas
A) always contain sand dunes. B) are typically not eroded by water. C) typically contain very little desert pavement. D) are eroded by both wind and water.