题目内容：

    When the great library of Alexandria burned, the story goes, one book was saved. But it was not a valuable book, and a poor man got it.

The book wasn't very interesting, but between its pages there was something very interesting indeed. It was a thin strip of vellum（羊皮纸）on which was written the secret of the "Touchstone".

The touchstone was a small pebble（鹅卵石）that could turn any common metal into pure gold.

The writing explained that it was lying among thousands and thousands of other pebbles that looked exactly like it. But the secret was this; The real stone would feel warm, while ordinary pebbles are cold.

So the man sold his house, bought a tent, camped on the seashore, and began testing pebbles. He knew that if he picked up normal pebbles and threw them down again because they were cold, he might pick up the same pebbles hundreds of times. So, when he felt one that was cold, he threw it into the sea. He spent a whole day doing this but none of them was the touchstone. Yet he went on and on this way. Pick up a pebble. Cold—throw it into sea. Pick up another. Throw it into the sea. The days stretched into months and the months into years.

One day, however, about midafternoon, he picked up a pebble and it was warm. He threw it into the sea before he realized what he had done. He was used to throwing each pebble into the sea as soon as he picked it up. So when the one he wanted came along, he still threw it away.

So it is with opportunity（机会）. Unless we are vigilant, it’s easy to fail to realize an opportunity when it is in hand and it's just as easy to throw it away.

1.According to the passage, the touchstone is a stone __________.

A.that you can touch

B.that can turn common metal into pure gold

C.that can bring you thousands of pebbles

D.that can bring you good luck

2.The man threw the touchstone into the sea because __________________.

A.it was cold

B.it was a normal pebble

C.he was too tired to hold it

D.he had formed a habit

3.How did the man probably feel when he realized it was the touchstone?

A.Excited. B.Moved. C.Interested. D.Regretted.

4.What's the meaning of the underlined word “vigilant” in the last paragraph?

A.Brave enough. B.Hard-working enough.

C.Careful enough. D.Strong enough.

5.What does the writer want to tell us?

A.Opportunity knocks but once.

B.Use it or lose it.

C.Where there is a will, there is a way.

D.Practice makes perfect.