Civil wars are fought between citizens of the same country and are fought within the country
内战发生在同一个国家的公民之间，并在该国内部进行。
If citizens of a country disagree about a policy or leader , they may fight a civil war
如果一个国家的公民对一项政策或领导人意见不一，他们可能会打一场内战
A world war is fought when countries form alliances and these alliances fight each other
当国家结成联盟，而这些联盟相互争斗时，就发生了世界大战。
A world war involves many or most of the world's most powerful countries
世界大战涉及世界上许多或大多数最强大的国家
There were two world wars in the 20th century
20世纪有两次世界大战
Terrorism involves groups of individuals rather than countries
恐怖主义涉及的是个人团体而不是国家
Terrorists may come from different countries and they may share the same beliefs
恐怖分子可能来自不同的国家，他们可能有相同的信仰
Terrorists often use car-bombings and suicide attacks on innocent people
恐怖分子经常使用汽车爆炸和自杀式袭击无辜的人。
A terrorist attack can happen at any time and any place
恐怖袭击随时随地都可能发生
An invasion occurs when one country attacks another
当一个国家攻击另一个国家时，侵略就发生了
It sends its army into the other country and attempts to take control
它派遣军队进入另一个国家并试图控制
One of the most famous invasions was when France invaded Russia in 1812
最著名的入侵之一是1812年法国入侵俄罗斯。
A defensive war occurs when a country is attacked and needs to defend itself
当一个国家受到攻击，需要自卫时，就会发生防御性战争
Most countries have a military so that it can defend itself if its attacked
大多数国家都有自己的军队，以便在受到攻击时能够自卫
When France invaded Russia in 1812 ， the Russian army fought a defensive war and defeated the French army
1812年法国入侵俄国时，俄国军队打了一场防御战，打败了法国军队。
测验：
Civil wars are often fought when one part of a country wants to become an independent country
当一个国家的一部分想要成为一个独立的国家时，内战经常发生。
Throughout history there have been many invasions
历史上有许多侵略
Sometimes two leaders will fight for power within a country and start a civil war
有时两个领导人会在一个国家内争夺权力，并引发内战
A cell is the basic unit of life
细胞是生命的基本单位
A cell is the smallest structure that can live and reproduce
细胞是能存活和繁殖的最小结构。
Cells grow and develop , reproduce and interact with the environment
细胞生长、发育、繁殖并与环境相互作用
Photosynthesis is one of the most important biological processes
光合作用是最重要的生物学过程之一。
It allows plants to capture and use light as an energy source
它允许植物捕捉和利用光作为能源。
Photosynthesis is the process of converting light energy into chemical energy
光合作用是将光能转化为化学能的过程。
A virus is a small biological structure that attacks and enters cells
病毒是一种攻击和进入细胞的小型生物结构
Once inside a cell , a virus reproduces and makes many copies of itself
一旦进入细胞，病毒就会繁殖并自我复制很多次
Some diseases such as influenza are viruses
有些疾病如流感是病毒
Bacteria are one-celled forms of life that are all around us
细菌是生活在我们周围的单细胞生物
Although they have a single-cell structure , they are very important
虽然它们是单细胞结构，但它们非常重要
Many bacteria are inside us and help us digest our food
许多细菌在我们体内帮助我们消化食物
DNA is a complex molecule that stores information and can make copies of itself
DNA是一种复杂的分子，它存储信息并能复制自身。
Nearly every cell in a person's body has the same DNA
人体内几乎每一个细胞都有相同的DNA。
The information stored in DNA tells cells what to do and is necessary for reproduction
储存在DNA中的信息告诉细胞该做什么，是繁殖所必需的
The influenza virus spreads through the air when people cough or sneeze
当人们咳嗽或打喷嚏时，流感病毒通过空气传播