请问补充材料第14篇第9题谢谢

inlovesummer

Passage 14

       Nearly a century ago, biologists found that if they

separated an invertebrate animal embryo into two parts

at an early stage of its life, it would survive and develop

as two normal embryos. This led them to believe that the

 (5)  cells in the early embryo are undetermined in the sense

that each cell has the potential to develop in a variety of

different ways. Later biologists found that the situation

was not so simple. It matters in which plane the embryo

is cut. If it is cut in a plane different from the one used

(10)        by the early investigators, it will not form two whole

embryos.

    A debate arose over what exactly was happening.

Which embryo cells are determined, just when do they-

become irreversibly committed to their fates, and what

(15) are the “morphogenetic determinants” that tell a cell

what to become? But the debate could not be resolved

because no one was able to ask the crucial questions

in a form in which they could be pursued productively.

Recent discoveries in molecular biology, however, have

(20)  opened up prospects for a resolution of the debate.

Now investigators think they know at least some of the

molecules that act as morphogenetic determinants in

early development. They have been able o show that,

   in a sense, cell determination begins even before an egg

(25) is fertilized. 

   Studying sea urchins, biologist Paul Gross found

that an unfertilized egg contains substances that func-

tion as morphogenetic determinants. They are located

in the cytoplasm of the egg cell; i.e., in that part of the

(30) cell’s protoplasm that lies outside of the nucleus. In the

unfertilized egg, the substances are inactive and are not

distributed homogeneously. When the egg is fertilized,

the substances become active and, presumably, govern

the behavior of the genes they interact with. Since the

(35) substances are unevenly distributed in the egg, when the

fertilized egg divides, the resulting cells are different

from the start and so can be qualitatively different in

their own gene activity.

       The substances that Gross studied are maternal

(40) messenger RNA’s --products of certain of the maternal

genes. He and other biologists studying a wide variety

of organisms have found that these particular RNA’s

direct, in large part, the synthesis of histones, a class

of proteins that bind to DNA. Once synthesized, the

(45) histones move into the cell nucleus, where section of

DNA wrap around them to form a structure that resem-

bles beads, or knots, on a string. The beads are DNA

segments wrapped around the histones; the string is the

intervening DNA. And it is the structure of these bea

 

9. Which of the following circumstances is most

  comparable to the impasse biologists encountered in

  trying to resolve the debate about cell determination

  (lines 12-18)?

  (A) The problems faced by a literary scholar who wishes

     to use original source materials that are written in

     an unfamiliar foreign language

  (B) The situation of a mathematician who in preparing a

     proof of a theorem for publication detects a

     reasoning error in the proof

  (C) The difficulties of a space engineer who has to

     design equipment to function in an environment in

     which it cannot first be tested

  (D) The predicament of a linguist trying to develop a

     theory of language acquisition when knowledge of

     the structure of language itself is rudimentary at best

  (E) The dilemma confronting a foundation when the

     funds available to it are sufficient to support one of

     two equally deserving scientific projects but not both

不明为什么选D. 谁可以把那句抽象的话明确点啊. 谁可以解释一下二者的是如何类比的呢.