关于历史发展的动力，下列选项中表述正确的是（　　）

【名词&注释】

科学技术(science and technology)、阶级斗争(class struggle)、过去时(past tense)、解放生产力(liberate the productive forces)、感受器(receptor)、潜伏期延长(prolonged latency)、动作电位潜伏期、动作电位幅度(action potential amplitude)、不一定(not always)

[单选题]关于历史发展的动力，下列选项中表述正确的是（　　）

A. 科学技术是直接的生产力
B. 阶级斗争是阶级社会发展的根本动力
C. 英雄与群众共同创造历史
D. 革命可以解放生产力

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学习资料：

[单选题]John________ he could improve his exam results, but he did not have enough time to study．

A. knows how
B. knows that
C. knew how
D. knew that

[单选题]_________

根据以下内容，回答下面的题目。Insure means to protect _51_ a loss of money．Most people can budget their money so that their income will cover expected expenses _52_ food, clothing, housing, and public services．But, there is no way to know _53_ who will suffer a crisis (危机) such as a serious illness, fire, flood, or a car accident．Such crises usually _54_ great expense．Even if people could predict crises, it would be hard to save enough money to _55_ the expenses．Insurance is a system _56_ a company collects money from many individuals and then pays certain expenses whenever one of those insured individuals is faced with a certain crisis．An insurance policy _57_ how much the insurance costs and how much the company will pay when a policy holder is faced with a certain crisis．There are many different kinds of insurance, _58_ hospital, motor-car and fire．Insurance can be rather expensive but most people buy insurance of some kinD．Insurance is something _59_ people buy and hope they will _60_ need．_________

A. never
B. ever
C. sometimes
D. often

[单选题]下列哪一种情形，委托代理不一定^{(not always)}终止 （　　）

A. 时效中断
B. 时效的延长
C. 时效的中止
D. 法定时效期间改变

[单选题]感受器的适应现象表现在持续刺激条件下（　　）

A. 动作电位发放频率变慢
B. 动作电位幅度^{(action potential amplitude)}变小
C. 动作电位发放频率变慢，幅度变小
D. 动作电位潜伏期延长
E. 动作电位发放频率变快

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