Question 20-30 7 O& R1 v! L8 F, ^" K( J
It is estimated that over 99 percent of all species that ever existed have become 6 [& i; E& r! g( e! Y$ Y
extinct. What causes extinction? When a species is no longer adapted to a changed 1 F4 u/ X: j, |, _1 G* [* K
environment, it may perish. The exact causes of a species’ death vary from situation
) e* C' i5 x& I3 D1 n9 q# }Line to situation. Rapid ecological change may render an environment hostile to a species./ @# D. ~8 _1 x9 S) c Y
For example, temperatures may change and a species may not be able to adapt. Food
, d7 x% J3 I3 K# @4 S) f' p- T1 z6 P% DResources may be affected by environmental changes, which will then cause problems
+ M% H. `' v, Y) t6 u+ ?+ B9 fFor a species requiring these resources. Other species may become better adapted to an
# b2 p* p& w r' v7 U8 JEnvironment, resulting in competition and, ultimately, in the death of a species.# R" G4 r! D+ T+ x7 P7 N
The fossil record reveals that extinction has occurred throughout the history of Earth.5 y9 ^- W" `' ?& W, r
(10) Recent analyses have also revealed that on some occasions many species became extinct / a! L; @* X) b7 x) f+ n7 i1 D
at the same time—a mass extinction. One of the best-known examples of mass extinction" M9 s0 \$ z" j& w
occurred 65 million years ago with the demise of dinosaurs and many other forms of life.8 P) n4 v. ]- r/ Q# U. p
Perhaps the largest mass extinction was the one that occurred 225 million years ago,5 Y# S" S6 n5 F. Q
When approximately 95 percent of all species died, Mass extinctions can be caused by
4 L6 j4 Z/ q6 I7 h6 _2 ^(15) a relatively rapid change in the environment and can be worsened by the close0 n ^3 x+ F/ `, n- O! P# {
interrelationship of many species. If, for example, something were to happen to destroy
^! \# \( d- h0 U much of the plankton in the oceans, then the oxygen content of Earth would drop,
: Q2 p) n* N& o affection even organisms not living in the oceans. Such a change would probably lead to a mass extinction.
4 H& O% t0 p5 p: g7 X$ @One interesting, and controversial, finding is that extinctions during the past 250% u( }/ m1 d: P: f3 k, `
Million years have tended to be more intense every 26 million years. This periodic / l$ S$ K0 R7 ?
extinction might be due to intersection of the Earth’s orbit with a cloud of comets, but
" O2 z2 f6 Y; j& k, Mthis theory is purely speculative. Some researchers have also speculated tat extinction; N* c- i+ E0 a# T
may often be random. That is, certain species may be eliminated and others may survive
2 g2 d3 ? Y. n J4 F {(25) for no particular reason. A species’ survival may have nothing to do with its ability or& g( S) m2 b+ l/ z/ k4 w
inability to adapt. If so, some of evolutionary history may reflect a sequence of essentially 9 |' E; j+ I9 ]7 Z2 ]
random events.
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, f9 S+ ]9 z: M. M6 l$ C" u! [* j/ E( J# w2 j4 a; X
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20. The word “it” in line 3 refers to
* w6 C3 |4 o# E# J (A) environment
4 l$ s% E) y, H9 b/ }0 N) t (B) species
- P7 q7 O1 P# g7 w (C) extinction
1 b1 `0 R v+ [% p; f3 E (D) 99 percent |/ \; F/ i( f& h5 z
21. The word “ultimately” in line 8 is closest in meaning to
+ X% p- A# B7 D. @ p+ ] (A) exceptionally( k2 y/ ~+ ]! a4 V
(B) dramatically" ~* g2 G6 }, Z; _9 n4 i
(C) eventually
n7 J- L Q, b- o: N9 }% G (D) unfortunately |
发表于 2012-8-14 22:34:35
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