单选题
What is the talk mainly about? A. The processes by which natural and cultured pearls are created. B. Some common misconceptions about the formation of pearls. C. The development of a technology to produce artificial pearls. D. A biological explanation of why pearls form in oysters.
【正确答案】
A
【答案解析】[解析] 6-11 [Professor (male)] Good morning, class. Let's get started. Q6 Uh, what I wanted to talk with you about today is the formation of pearls. More specifically, I'd like to compare two ways that pearls form: naturally...and with human help. Let's start by talking about how pearls form naturally. I suppose most of you already have an idea how pearls develop in oysters, but let's just go over the process to clear up any misconceptions you may have and make sure we're all working with the same information. OK? Natural pearls begin with a nucleus. [Student A (male)] You mean like a grain of sand or something? I've heard that pearls form around a tiny grain of sand that gets into an oyster's shell. Is that true? [Professor] Well, the nucleus could be a grain of sand...a tiny parasite...a particle of plant material--just something that finds its way into an oyster's shell when it's open for the purpose of feeding or respiration. But the thing is...this nucleus, whatever it is, it irritates the oyster. When this foreign particle becomes lodged between the oyster's shell and its mantle--urn, which is the membrane that surrounds its soft body--when this happens, the oyster responds to protect itself from the particle. Q7 So what it does is coat the particle in nacre. And what's nacre? Nacre is that calcium carbonate material that covers the inside of an oyster shell. You know...the shiny stuff. Over and over, the original nucleus gets covered in layers of nacre. And when these layers build up enough, they form what we recognize as a pearl. Q8 If you looked at a cross section of a natural pearl, you'd see a series of concentric circles, like the growth rings of a tree, And it all started with that tiny foreign particle that got inside the oyster's shell. [Student B (female)] So...natural pearls are formed just by accident, really? I mean, the oyster doesn't intentionally create the pearl for any reason? [Professor] That's right. Pretty interesting, huh?The creation of natural pearls is just a response to an irritating particle. Um...also, since they form under such specific circumstances, natural pearls are very rare, as you can imagine. Q9 But what's even rarer is a natural pearl that's perfectly round. And since a perfectly round pearl is nicer to look at than an irregular one, um...round natural pearls tend to be really expensive. [Student B] Q11 But nowadays pearls are more affordable in general...because we have the ability to manufacture them, right? And it's easy to make them perfectly round. [Professor] Well, yes. Pearls that form with help from humans are known as cultured pearls. But, uh, manufacture probably isn't the right term. It's not like we're building them in a factory.Cultured. pearls form in much the same way as natural pearls--I mean, we still rely on oysters to create them. [Student A] So how does the process work...the process of making a cultured pearl? Do people actually, um, insert nuclei into oysters? [Professor] That's right. The technology to create cultured pearls was developed during the beginning of the twentieth century. And it basically works just like you described it. Q10 A nucleus is surgically implanted inside an oyster's shell. Um, this nucleus...they're usually made from a piece of the shell of an oyster... or another shelled mollusk. And...well, once the nucleus is in, the oyster pretty much takes over from there.The process is essentially the same as natural pearl formation. Nacre coats the nucleus and a pearl forms. Uh, aside from the fact that cultured pearl formation involves humans inserting a nucleus, the main difference between natural and cultured pearl formation is the size of the nucleus. Natural pearls begin with a rather tiny particle, and the bulk of the pearl is made of nacre--layers and layers of it. Well, in cultured pearls the opposite is true. Most of a cultured pearl's mass is made up of the artificial nucleus, and the nacre coating on the outside is just a thin layer. Because of this, the nuclei of cultured pearls are really more like beads than tiny particles. They're nearly the same size and shape as the final pearl. Q8 If you looked at a cross section of a cultured pearl, it'd seem pretty different from the cross section of a natural pearl. As I said before, natural pearls have tiny growth rings. You know, just like a tree has growth rings every year as it gets bigger, or uh, shells get growth rings as they develop. But, uh, if you looked at the cross section of a cultured pearl, you wouldn't see any growth rings, just a thin layer around the nucleus. Most of the interior would just be solid nucleus bead. Got it?
单选题
What role does nacre play in the formation of pearls? A. It provides the nucleus around which a natural pearl develops. B. It is a calcium coating that protects the outside of the oyster's shell. C. It penetrates the membrane covering the oyster's soft body. D. It is the luminous substance that forms the pearl's outer layers.
【正确答案】
D
【答案解析】
单选题
Why does the professor mention growth rings in trees? A. To compare natural pearl development to something the students are familiar with. B. To show the contrast between growth rings in pearls and those in living organisms. C. To highlight a process that occurs more frequently in cultured pearls than natural pearls. D. To explain why it is easy for scientists to determine the age of natural pearls.
【正确答案】
A
【答案解析】
单选题
What does the professor say about round natural pearls? A. They do not look as attractive as they used to. B. They have become more common than they once were. C. People are willing to pay large amounts of money for them. D. No one is entirely sure how they are formed.
【正确答案】
C
【答案解析】
单选题
What is usually used as the nucleus for a cultured pearl? A. A fragment of mollusk shell. B. A large grain of sand. C. A particle of plant material. D. A tiny parasite.
【正确答案】
A
【答案解析】
单选题
Why does the professor say this: A. To make sure the woman does not misunderstand how cultured pearls are made. B. To indicate that the manufacture of cultured pearls is still an inexact science. C. To express uncertainty about the process used to manufacture cultured pearls. D. To define the technical differences in the formation of natural and cultured pearls.