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问题 问答题

骑自行车下坡时，人不蹬脚踏板，车速也会越来越快．请你从能量转换的角度解释这个现象．

答案

当人骑自行车下坡时，总质量不变，高度变小，故其重力势能变小；同时其速度变快，即动能变大，故人骑自行车下坡时重力势能逐渐转化为动能，使人和自行车的重力势能减小，动能增加，所以车速度越来越快．

选择题
物体以12 m/s的初速度在水平冰面上做匀减速直线运动，它的加速度大小是0.8m/s2，经20s物体发生的位移是（   ）

A．80m

B．90m

C．100m

D．110m
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单项选择题

In the following essay, each blank has four choices. Choose the best answer and write down on the answer sheet.

In security, an (1) is a form of possible loss or harm in a computing system; examples of exposures are unauthorized disclosure of data, modification of data, or (2) of legitimate access to computing. A vulnerability is a weakness in the security system that might be exploited to cause loss or harm. A human who exploits a vulnerability perpetrates an attack on the system. Threats to computing systems are circumstances that have the potential to cause loss or harm; human attacks are examples of threats, as are natural disasters, inadvertent human errors, and internal hardware or software (3) . Finally, a control is a protective measure an action, a device, a procedure, or a technique - that reduces a vulnerability. The major (4) of computing systems are hardware, software, and data. There are four kinds of threats to the security of a computing system: interruption, interception, modification, and (5) . The four threats all exploit vulnerabilities of the assets in computing systems.

3（）

A．problems

B．flaws

C．defect

D．shortcoming
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