[阅读理解] Long, long ago there was no zero. To write the number sixty-three people wrote 6 3. To write six hundred and three, people wrote 6 3. The space between six and three was there to mean." not any "tens. Sometimes people did not remember the space. It was hard to see and read. Later people used a dot to hold the space. Six hundred and three looked like this 6.3. But the dot was hard to see. So people put a circle around it like this 6⊙3. Then people could see the dot. They remembered the space. At last, only the circle around the dot was used. It was like a zero. This is one story of how the zero came to be used. Now zero has many important uses. Zero tells how many. Can you tell some other ways of using zero?
4. People used circles ________. A.to remember ways B.to remember numbers C.not to forget the space D.to mean nothing