单项选择题

Once you decide to choose a topic to write an effective paragraph, the first key to write a good topic sentence. The topic sentence does two things: it tells the reader what the topic of the paragraph is, and it says something about that topic. What the topic sentence says about the topic is called the controlling idea; it is the point of the paragraph, and every other sentence in the paragraph must be related to this point.
The topic sentence should be the first sentence in the paragraph unless there is some special reason for placing it somewhere else. When the topic sentence is the first sentence of the paragraph, the reader immediately knows what you intend to discuss. In addition, you have the topic sentence to refer to as you make each of the other sentences in the paragraph, so that you can be sure that each of those sentences refers to your subject and to your controlling idea. Sometimes experienced writers put the topic sentence at the end of the paragraph or even in the middle to create some kind of special impression; but while you are still learning to write effective paragraphs, it is safest to put the topic sentence at the beginning unless you have a special reason for doing something else.
A good topic sentence is a clear, definite statement with a limited and special topic and a limited and specific controlling idea. It is usually placed at the beginning of the paragraph. Now that you have had some practice in identifying and constructing good topic sentences, we are ready to look at some of the basic principles that govern the composition of effective paragraphs.

The topic of this passage probably is（）

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