在数据库系统中，当总体逻辑结构改变时，通过改( )，使局部逻辑结构不变，从而使建立在局部逻辑结构之上的应用程序也保持不变，称之为数据和程序的逻辑独立性。

A．应用程序

B．逻辑结构和物理结构之间的映像

C．存储结构

D．局部逻辑结构到总体逻辑结构的映像

Smoking
Since 1939, numerous studies have been conducted to determine whether smoking is a health hazard. The trend of the evidence has been consistent and indicates that there is a serious health risk. Research teams have conducted studies that show beyond all reasonable doubt that tobacco smoking is associated with a shortened life expectancy.
Cigarette smoking is believed by most research workers in this field to be an important factor in the development of cancer of the lungs and cancer of the throat and is believed to be related to cancer of some other organs of the body. Male cigarette smokers have a higher death rate from heart disease than non-smoking males. Female smokers are thought to be less affected because they do not breathe in the smoke so deeply.
Apart from statistics, it might be helpful to look at what tobacco does to the human body. Smoke is a mixture of gases, vaporized chemicals, minute particles of ash and other solids. There is also nicotine, which is powerful Poison, and black tar. As smoke is breathed in, all those components from deposits on the membranes of the lungs. One point of concentration is where the air tube and bronchus divides. Most lung cancer begins at this point.
Filters and low tar tobacco are claimed to make smoking to some extent safer, but they can only slightly reduce, not eliminate the hazards.

It is easy to determine whether smoking is hazardous.

A．Right

B．Wrong

C．Not mentioned