生存斗争是下列哪种原因引起的？（ ）
A．遗传和变异
B．适应性
C．过度繁殖
D．生物的多样性
自然界中各种生物普遍具有很强的繁殖能力，从而能产生大量的后代，即过度繁殖．而生物赖以生存的食物和空间是有限的，生物为了获取食物和空间，要进行生存斗争．自然界中生物个体都有遗传和变异的特性，只有那些具有有利变异的个体，在生存斗争中才容易生存下来，并将这些变异遗传给下一代，而具有不利变异的个体被淘汰．自然界中的生物，通过激烈的生存斗争，适应者生存下来，不适应者被淘汰掉，这就是自然选择，过度繁殖是导致生存斗争的原因．
故选：C
Teamwork is just as important in science as it is on the playing field or in the gym. Scientific investigations (调查) are almost always carried out by teams of people working together. Ideas are shared, experiments are designed, data are analyzed, and results are evaluated and shared with other investigators. Group work is necessary, and is usually more productive than working alone. Several times throughout the year you may be asked to work with one or more of your classmates. Whatever the task your group is assigned, a few rules need to be followed to ensure a productive and successful experience. What comes first is to keep an open mind, because everyone’s ideas deserve consideration and each group member can make his or her own contribution. Secondly, it makes a job easier to divide the group task among all group members. Choose a role on the team that is best suited to your particular strengths. Thirdly, always work together, take turns, and encourage each other by listening, clarifying, and trusting one another. Mutual （相互的）support and trust often makes a great difference. Activities like investigations are most effective when done by small groups. Here are some more suggestions for effective team performance during these activities: Make sure each group member understands and agrees to the task given to him or her, and everyone knows exactly when , why and what to do; take turns doing various tasks during similar and repeated activities; be aware of where other group members are and what they are doing so as to ensure safety; be responsible for your own learning, though it is by no means unwise to compare your observations with those of other group members. When there is research to be done, divide the topic into several areas, and this can explore the issue in a very detailed way. You are encouraged to keep records of the sources used by each person, which helps you trace back to the origin of the problems that may happen unexpectedly. A format for exchanging information (e.g., photocopies of notes , oral discussion, etc.) is also important, for a well-chosen method not only strengthens what you present but also makes yourself easily understood. When the time comes to make a decision and take a position on an issue, allow for the contributions of each member of the group. Most important of all, it is always wise to make decisions by compromise and agreement. After you’ve completed a task with your team, make an evaluation of the team’s effectiveness — the strengths and weaknesses, opportunities and challenges.
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