Certainly, there have been periods in Earth's history when mass extinctions have occurred. The extinction of the dinosaurs was caused by some physical event, either climatic or cosmic. There have also been less dramatic extinctions, as when natural competition between species reached an extreme conclusion. Only .01 percent of the species that have lived on Earth have survived to the present, and it was largely chance that determined which species survived and which died out.

However, nothing has ever equaled the magnitude and speed with which the human species is altering the physical and chemical world and demolishing the environment. In fact, there is wide agreement that it is the rate of change humans are inflicting, even more than the changes themselves, that will lead to biological devastation. Life on Earth has continually been in flux as slow physical and chemical changes have occurred on Earth, but life needs time to adapt-time for migration and genetic adaptation within existing species and time for the proliferation of new genetic material and new species that may be able to survive in new environments.

42. What does the passage mainly discuss?

(A)The causes of the extinction of the discuss?
(B)The variety of species found in tropical rain forests.
(C) The impact of human activities on Earth's ecosystems
(D) The time required for species to adapt to new environments

43. The word "critical" in line 1 is closest in meaning to

(A) negative
(B) essential
(C)interesting
(D) complicated

44. The word "jolting" in line 5 is closest in meaning to

(A) predicted
(B) shocking
(C)unknown
(D) illuminating

45. The author mentions the reduction of the variety of species on Earth in lines 11 - 12 to suggest that

(A) new habitats can be created for species
(B)humans are often made ill by polluted water
(C) some species have been made extinct by human activity
(D) an understanding of evolution can prevent certain species from disappearing

46. The author mentions all of the following as examples of the effect of humans oil the world's ecosystems EXCEPT

(A) destruction of the tropical rain forests
(B) habitat destruction in wetlands
(C)damage to marine ecosystems
(D)the introduction of new varieties of plant species

47.The author mentions the extinction of the dinosaurs in the second paragraph to emphasize that

(A)the cause of the dinosaurs extinction is unknown
(B)Earth's climate has changed significantly since the dinosaurs' extinction
(C)not all mass extinctions have been caused by human activity
(D) actions by humans could not stop the irreversible process of a species' extinction

48.The word "magnitude" in line 20 is closest in meaning to

(A) concern
(B) determination
(C)carelessness
(D) extent

49. According to the passage, natural evolutionary change is different from changes caused by humans in that changes caused by humans

(A) are occurring at a much faster rate
(B) are less devastating to most species
(C)affect fewer ecosystems
(D) are reversible

50. With which of the following statements would the author be most likely to agree?