| Rocks which have solidified directly
from molten materials are called igneous rocks. Igneous rocks are commonly
referred to as primary rocks because they are the original source of material
found in sedimentaries and metamorphics. Igneous rocks compose the greater part
of the earth’s crust, but they are generally covered at the surface by a
relatively thin layer of sedimentary or metamorphic rocks. Igneous rocks are
distinguished by the following characteristics: (1) they contain no fossils; (2)
they have no regular arrangement of layers; and (3) they are nearly always made
up of crystals. Sedimentary rocks are composed largely of minute fragments derived from the disintegration of existing rocks and in some instances from the remains of animals. As sediments are transported, individual fragments are assorted according to size. Distinct layers of such sed A. Granite. B. Shale. C. Slate. D. Limestone. [单项选择]From which college did Jefferson graduate
A. Washington College. B. College of William and Mary. C. The university of Virginia. D. Shadwell College. [单项选择]Our research has focused on a drug which is so （ ） as to be able to change brain chemistry.
A. powerful B. influential C. monstrous D. vigorous [单项选择]Cancer
1. Cancer, which may arise from any type of cell and in any body tissue, is not a single disease but a large number of diseases classified according to the tissue and type of cell of origin. Several hundred such classes exist, constituting three major subtypes; sarcomas, carcinomas, leukemias and lymphomas.
2. A number of factors produce cancer in a proportion of exposed individuals. Among these factors are heredity, viruses, ionizing(离子)radiation, chemicals, and alterations in the immune system. For a long time these various factors seemed to work in different ways, but now researchers are studying how they might interact.
3. More than 1,350,000 new cases of cancer occur in the United States each year. It is the second leading cause of death in the nation, accounting for about 550,000 deaths annually. The incidence of cancer varies enormously among different geographic areas. The age-adjusted death rate from all cancers in males is 310. 9 per 100,000 in Luxembourg (the hi
我来回答: 提交
|
