中心句和内容的关系不能像炸鸡排的纸包装袋，只包住一部分内容；
也不能像薯片，打开后发现内容只占中心句的一小部分，剩下很多空间未讨论；
而应该像真空包装的卤鸡腿，中心句把内容完全包含，同时没有未讨论的空间。
由于题目本身很笼统，因此有的同学也想让body段覆盖更大的范围，于是写下了这样的段落中心句：
To start with, it is immoral for governments to focus on their own people.
首先focus并不等同于题目中的only help，缺少了“仅”的含义。immoral涵盖面太广，后面得解释是哪方面缺德。
另一个反例：
On one hand, it would be extremely selfishfor governments to exclusively focus on their own residents.
用extremely selfish代替immoral并加上exclusively之后好了很多，但往下论述时发现，这个topic中governments包括了所有国家，然而欠发达国家是不可能帮助别人的。那下一句就得补充解释Developed countries should spare their excessive resources to facilitate less developed countries in need so that the refugees in those countries could survive.
这时候还有咱developing country没讨论。不能在developed countries后面硬塞and developing countries，会让考官觉得以偏概全，必须另写一句。这时再加上分别的举例/深入讨论，这段肯定会过长。
因此，一个好的topic sentence应该直接从细节入手：
On one hand, it would be inhumane and inconsiderate for developed countries to reservesuperfluous resources exclusively for their own residents.
P.S.: 这里用superfluous（过剩的）或者excessive（过量的）比较好，而redundant（冗余的），unneeded（不需要的），dispensable （可有可无的）在这里意思都不恰当。自行体会。
这样的topic sentence就让后续部分容易展开多了。首先给出了具体的缺陷（inhumane and inconsiderate），划定了国家范围（developed countries）并把可以用来援助的资源（superfluous resources）都限定好了，那下一句顺理成章地解释一下：
If developed countriescapable of helping chose to ignore the calls for aid and left those desperate people in despair, their inaction would possibly lead to mortality and their governments would be subject to the accusation of irresponsibility and indifference.
这句虚拟语态的解释句也起到了举例的作用，接下来可以深入讨论:
Meanwhile, practicing generosity not only answers to our nature of sympathy and respect for life, but also arouses and cascades the virtue of helpfulness among the general public. A positive example is therefore established by the government for those who are affluent yet hesitant to help theless fortunate.
