请阅读下列短文并找出主题句
Africa――The fight against starvation in six West African nations is being hampered by the rainy
season. Heavy rains are turning the dirt roads into muddy rivers. Relief will have to wait for a dry spell which seems nowhere in sight. The weather bureau is maintaining silence for fear of causing increased alarm.
Londons weather is strange. It can rain several times a day; each time the rain may come suddenly after the sun is shining brightly. The air is damp and chill right through July. On one March afternoon Hampton Heath last year， it rained three times， there was one hailstorm， and the sun shone brilliantly―― all this within two hours time. It is not unusual to see men and women rushing down the street on a sunny morning with umbrellas on their arms. No one knows what the next few moments will bring.
Surviving in the jungle is a science. The jungle people have become perfect in this science， and you can too. Learn as much as you can about what to expect in the jungle. Make sure you have the jungle. Make sure you have the right equipment. Then no part of the jungle will seem completely unfriendly or frightening. In fact you will be able to "live off" it for a long time.
Perhaps more than any other people Americans have come to depend on their cars. The family car has been a common thing since early 20th century， and it has changed American life. Many people have moved outside of the large cities to the suburbs Some Americans spend two hours a day or more in their cars going to work and home again. Cars have become the means of transportation for most Americans.
很显然，上面四篇短文的第一句都是主题句。
(2)主题句在段尾。
虽然英语段落的主体旬在大多数情况下都置于段落之首，但也有时出现在段落之末。因此，如果第一句话不是概括性的或综合性的话，最好快速读一读段落的最后一个句子，看看它是否具备主题句的特征：如果它具备主题句的特征，短文的主题思想就很容易确定了。
一般说来，当一种观点不易向人解释清楚，或不易被人接受时，主题句便会到段落的末尾才出现。当作者想要说服读者听信其言时，一般采用此格式组织句子。通常似乎有这样一种倾向：人们只对自己了解的新思想新观点才考虑接受它。换句话说，只有在听了对某一新思想新观点的细节解释说明之后，人们才可能接受它。有些文章以列举事实依据开头，通过论证，最后讲出作者自己所阐述的核心论点。
在做阅读理解试题时，要充分利用引出结论的信号词，如clearly，plainly;so，therefore，thus，consequently;In conclusion，in short等来确定主题句的位置在段尾。当无明显的此类信号同时，可添加一个引出结论的信号词以确定主题句是否是段落的最后一句话。
例如：
Americans might be embarrassed because their Japanese friends are so formal with them. Japanese might feel insulted because American acquaintances greet them casually. Still， the forms of greeting in both countries only show respect for others. It just happens that Americans and Japanese have a different way of looking at human relationships and thus have a different way of showing respect.
This passage mainly tells us_________.
A. how Americans show respect to others
B. how Japanese show respect to their American acquaintances
C. Americans and Japanese have different ways of showing respect
D. Japanese are not satisfied with American casual behavior
这一篇短文前几句只讲述了具体情况，均不具备主题句的特征，只有最后一句话是概括性的、总结性的，因此本段的最后一句是主题句，故正确答案是D.