历年GRE考试真题精选系列六十三

会计考友

The primary method previously
　　used by paleontologists to estimate
; A, w" P% b' {8 ]2 F0 ]; A　　climatic changes that occurred during
) e; I7 U: x- e' y# L- i　　Pleistocene glacial cycles was the
1 d* \/ B9 W0 Z' r( |( s  } (5) determination of 18/O16/O ratios in
1 H" e# A- G0 b( r% G- m$ A　　calcareous fossils. However, because
1 q* L1 K) G/ s# F$ E' a　　this ratio is influenced by a number
　　of factors, the absolute magnitude
　　of the temperature difference between
  L3 L8 k( S* y3 [(10) Pleistocene glacial and interglacial
) s" B/ [+ f; E7 e( B& V. K　　cycles could not be unequivocally
/ b, s: E# G' H$ D& c3 |　　ascertained. For example, both
/ K9 W' r9 p, Y1 o7 b1 p0 l! M! H　　temperature fluctuations and isotopic
　　changes in seawater affect the
1 \  Y# x5 |- h! Y9 w, q4 V(15) 18O/16O ratio. And. since both factors
　　influence the ratio in the same
　　direction, the contribution of each
　　to the 18O/16O ratio cannot be
. r3 [6 f% v/ P* |8 Q- I7 e　　determined.
0 E& b7 _' d7 C1 W0 A' `(20) 　Fortunately, recent studies indicate
　　that the racemization reaction of
　　amino acids can be used to determine
　　more accurately temperatures that
3 o' n9 l9 u- O' @, T& I　　occurred during Pleistocene glacial
(25) cycles. Only L-amino acids are usually
9 O" K8 B) ]. o3 @! ]9 F/ i2 Z　　found in the proteins of living
　　organisms, but over long periods of
　　geological time these acids undergo
　　racemization, producing D-amino acids,
(30) which are not found in proteins. This
　　reaction depends on both time and
　　temperature thus, if one variable is
3 `2 `) l! A& g/ ~　　known, the reaction can be used to
　　calculate the other.
  
% \( W" k! V9 JIt can be inferred from the passage
that determination of the temperatures
/ O$ L  [" I  F( d7 U9 N# u( b$ rmentioned in line 18 through 18O/16O
- K, ^9 Q5 n/ X! }* V" U4 H" ?ratios and determination through
racemization reactions both require
which of the following?
Calcium deposits known to be from
' o# V' f1 L$ z! |: j4 IPleistocene seas
1 I/ U+ J$ t" D& Y* l5 f' x Proteins containing both L-amino
acids and D-amino acids
Glacial debris from both before and
after the Pleistocene period
2 q/ P7 T" l: [6 y$ n Fossil material from organisms living
1 M1 N7 J& L$ J7 nduring the Pleistocene period
; B2 q: i, |5 m6 w& f3 a" h Proteins containing both amino acids
' f1 u4 g0 U/ v& Cand 18O
( A5 }/ N9 g4 H% O" C0 A2 {  
- L5 k8 E* `% W& u# S————————————————————————
$ n+ G+ n6 V, `7 O* j答案：(D)

会计考友

The passage suggests that the 18O/ 16O/
- A3 i, _; T% n5 _ratio
determine the 18O/ 16O ratio in living
animals as well as in fossil remains
locate a greater number of calcareous
  r( l5 B0 C1 P5 o" T4 Nfossils from the Pleistocene glacial and
interglacial cycles
locate the factors other than temperature
" X$ r# i7 N7 jfluctuations and isotopic changes in
seawater that affect the 18O/16O ratio
# H0 K! P; h) D/ h. q arrive at more exact determinations of
which amino acids are found in the
proteins of living organisms
isolate the relative effects of temperature
! g6 U) G9 D# b8 U' d) j) i2 c7 ^fluctuations and isotopic changes in
seawater on 18O/16O ratios
3 ^3 g$ P# r$ N! z; f  
4 X" k  e0 V( s+ D+ Q————————————————————————
' s, I) Y3 s4 W' `  X1 L答案：(E)
  
$ k; c' H4 i# \3 e4 e* f9 y?　The information in the passage can
! A5 U, M! q* ]% gbe used to answer which of the following
3 k3 [+ ^3 J  N, ^6 K$ pquestions?
! j5 h. T% a. ~9 X　Ⅰ Do temperature variations and isotopic
- I- U2 X. V  o; Tchanges in seawater cause the 18O/16O
ratio to shift in the same direction?
; R2 G) B, P8 C　Ⅱ What are the methods used to determine
5 K" U* A1 e5 N4 q; ~the18 O/16O ratio?
　Ⅲ Is the study of raceminzation reactions
useful in estimating climatic changes
that occurred during Pleistocene glacial
cycles if only one of the two important
3 ]( y$ k  F7 o# |1 [9 r4 i# qvariables is known?
! z: H6 K$ h/ g Ⅰonly
0 T$ K& f6 a2 y6 n; Y5 x8 ^+ b Ⅰand Ⅱ only
& C, E  U6 }/ l. L Ⅰand Ⅲ only
Ⅱand Ⅲ only
Ⅰ,Ⅱ,and Ⅲ
  
————————————————————————
3 v( A. h  a, Y. p; G$ p% ]答案：(C)