土的孔隙比表示孔隙体积与（）体积之比。A、固体颗粒 B、液体 C、气体 D、固体颗粒

She made a quick ____ from her illness and was soon back at work.[ ]

A. decision

B. recovery

C. change

D. promise

The most popular organic gem is the pearl. A pearl is the response of a marine mollusk to the presence of an irritating impurity accidentally introduced into its body; a cultured pearl is the result of the intentional insertion of a mother-of-pearl bead into a live mollusk. Whether introduced accidentally or intentionally, the pearl-making process is the same: the mollusk coats the irritant with a substance called nacre. Nacre is composed chiefly of calcium carbonate. Because very few natural pearls are now on the market, most pearls used in fine jewelry are cultured. These include "Biwa" pearls and most other freshwater pearls. Cultured pearls are not easily distinguished from natural pearls except by an expert.

Cultured pearl is formed by________.

A．insertion of a pearl into a live mollusk

B．an oyster into which a piece of grit has been placed

C．putting in a live mollusk

D．placing a bead into culture

电液执行机构与气动执行机构比较，气动执行结构的优点是（）。

A、噪声低

B、传动推力大

C、无滞后

D、本质安全

A bite of a cookie containing peanuts could cause the airway to constrict fatally. Sharing a toy with another child who had earlier eaten a peanut butter and jelly sandwich could raise a case of hives. A peanut butter cup dropped in a Halloween bag could contaminate the rest of the treats, posing an unknown risk.

These are the scenarios that "make your bone marrow turn cold" according to L. Val Giddings, vice president for food and agriculture of the Biotechnology Industry Organization. Besides representing the policy interests of food biotech companies in Washington, D. C., Giddings is the father of a four-year-old boy with a severe peanut allergy. Peanuts are only one of the most allergenic foods; estimates of the number of people who experience a reaction to the beans hover around 2 percent of the population.

Giddings says that peanuts are only one of several foods that biotechnologists are altering genetically in an attempt to eliminate the proteins that do great harm to some people’s immune systems. Although soy allergies do not usually cause life-threatening reactions, the scientists are also targeting soybeans, which can be found in two thirds of all manufactured food, making the supermarket a minefield for people allergic to soy. Biotechnologists are focusing on wheat, too, and might soon expand their research to the rest of the "big eight" allergy-inducing foods: tree nuts, milk, eggs, shellfish and fish.

Last September, for example, Anthony J. Kinney, a crop genetics researcher at DuPont Experimental Station in Wilmington, Del., and his colleagues reported using a technique called RNA interference (RNAi) to silence the genes that encode p34, a protein responsible for causing 65 percent of all soybean allergies. RNAi exploits the mechanism that cells use to protect themselves against foreign genetic material; it causes a cell to destroy RNA transcribed from a given gene, effectively turning off the gene.

Whether the public will accept food genetically modified to be low-allergen is still unknown. Courtney Chabot Dreyer, a spokesperson for Pioneer Hi-Bred International, a subsidiary of DuPont, says that the company will conduct studies to determine whether a promising market exists for low allergen soy before developing the seeds for sale to farmers. She estimates that Pioneer Hi-Bred is seven years away from commercializing the altered soybeans.

Doug Gurian-Sherman, scientific director of the biotechnology project at the Center for Science in the Public Interest—a group that has advocated enhanced Food and Drug Administration oversight for genetically modified foods—comments that his organization would not oppose low-allergen foods if they prove to be safe. But he wonders about "identity preservation" a term used in the food industry to describe the deliberate separation of genetically engineered and no nengineered products. A batch of nonengineered peanuts or soybeans might contaminate machinery reserved for low-allergen versions, he suggests, reducing the benefit of the gene-altered food. Such issues of identity preservation could make low-allergen genetically modified foods too costly to produce, Chabot Dreyer admits. But, she says, "it’s still too early to see if that’s true. \

From the text, we can know that RNAi（）

A. can deprive cells of certain mechanism

B. can protect cells against foreign genetic material

C. can be effective on 34 kinds of genes

D. can cause soybean allergies

过热器热偏差主要是受热不均匀和蒸汽流量不均匀造成的（）