Question 20-30 % F0 d$ ]- W& T' i/ a' e
It is estimated that over 99 percent of all species that ever existed have become
( J" F% a/ }- F' {9 ^) wextinct. What causes extinction? When a species is no longer adapted to a changed
9 Y+ F( }7 v' H- Z8 u3 h2 Venvironment, it may perish. The exact causes of a species’ death vary from situation % {8 S1 f `3 C' n; f" V, _
Line to situation. Rapid ecological change may render an environment hostile to a species.
5 H$ V7 O- `6 z5 d. f, ^7 [5 rFor example, temperatures may change and a species may not be able to adapt. Food
" ?) \# R7 r% X# T+ W" DResources may be affected by environmental changes, which will then cause problems5 J/ S& o9 y. g. V, X5 v) _2 x0 M; w" B
For a species requiring these resources. Other species may become better adapted to an
0 V1 n5 U! t3 A0 j! MEnvironment, resulting in competition and, ultimately, in the death of a species.7 d% |. _% z& }2 T
The fossil record reveals that extinction has occurred throughout the history of Earth.
+ Y4 i V. J/ ]) H5 Z" W' f$ T(10) Recent analyses have also revealed that on some occasions many species became extinct
) @# \5 B' Q( E9 o at the same time—a mass extinction. One of the best-known examples of mass extinction
2 W' h: Z! j$ I1 }2 S occurred 65 million years ago with the demise of dinosaurs and many other forms of life.
w/ i, \* k u7 V+ w1 g$ i Perhaps the largest mass extinction was the one that occurred 225 million years ago,
& ^0 Q( B8 G9 z( v' X# u When approximately 95 percent of all species died, Mass extinctions can be caused by" J5 m |3 v0 r F$ M' R
(15) a relatively rapid change in the environment and can be worsened by the close
* X8 e# R) K& s q# D& G/ N$ s. A interrelationship of many species. If, for example, something were to happen to destroy
* X' \: \: @; a/ J9 s much of the plankton in the oceans, then the oxygen content of Earth would drop,
% \3 {7 S$ Z7 s affection even organisms not living in the oceans. Such a change would probably lead to a mass extinction.
- g4 U* g9 K& ~One interesting, and controversial, finding is that extinctions during the past 250
; z# V' R: W) Z) f2 y. a& y Million years have tended to be more intense every 26 million years. This periodic
2 w; L, f% @1 k* @9 F+ Lextinction might be due to intersection of the Earth’s orbit with a cloud of comets, but+ w; D' d7 B4 ~" C8 F) z3 W. r
this theory is purely speculative. Some researchers have also speculated tat extinction% r! i9 V. y( h. `+ N ~% D
may often be random. That is, certain species may be eliminated and others may survive3 t. l6 U, H5 i1 X
(25) for no particular reason. A species’ survival may have nothing to do with its ability or Z& Q. B4 o6 C* n
inability to adapt. If so, some of evolutionary history may reflect a sequence of essentially
- r8 E. w3 j, f! {; U* g$ frandom events.
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8 t' X5 {4 y( ~5 t7 R, k5 u/ }1 A. n
20. The word “it” in line 3 refers to
# ^4 Q! R0 H3 ~3 l, ~. l8 V (A) environment
& q' ?6 Q# F0 O0 D (B) species
: d+ h, m9 e) X, K% n (C) extinction/ \ t5 g. ~( Q/ _
(D) 99 percent p$ z. x* b" E: ^9 }
21. The word “ultimately” in line 8 is closest in meaning to
2 W/ x3 l0 Z/ U% b (A) exceptionally; @# j6 l4 K$ j3 y
(B) dramatically, v ]8 L; a3 C7 x4 J# j
(C) eventually
; K: d. g; C$ P- [$ t (D) unfortunately |
发表于 2012-8-14 22:34:35
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