题目内容：

请仔细阅读前四个小题, 将有关地沟油(swill oil)的说明与下列A、B、C、D四个标题配对，然后回答第5个小题。

1.Put a drop of oil on a piece of paper，burn it and listen to how it sounds.Good oil usually burns quietly. Swill oil may produce a strange sound，which means there is too much water in it.

2.Put a drop of oil in your hand and put your hand near your nose to smell. Swill oil always has an unpleasant smell.

3.Using a chopstick，collect a drop of oil and taste it.Swill oil may taste sour and hot.

4.Ask the seller about which factory the oil is from and ask for a quality certificate(证明)．

5.What's this passage mainly about?

A. How to taste the swill oil.    B. How to smell the swill oil.

C. How to check the swill oil.    D. How to eat the swill oil.