A、They think exactly the same way. B、They are not physically separated. C、They share most of their vital organs. D、They make dec

Imagine this: you wake up each morning to find your sister lying beside you. To get dressed and tie your shoes, you use one hand and she uses another. You do everything out together, even sitting on the same chair at lunch and riding on the same bicycle. That’s what life is like for six-year-old Betty and Abby. Like most twins, the two girls look very much alike. But unlike most twins, Betty and Abby share parts of the same body. Twins like Betty and Abby are rare. Only about 40 sets are born in the United States each year. Few survive as long as Betty and Abby. That’s because twins often share vital organs, like a heart or brain. The shared organs are often badly shaped and may not be strong enough to support both twins. But Betty and Abby each has her own head, heart and stomach which function normally. They have three or four lungs which provide plenty of oxygen for both twins. Most of their completely shared organs he below the waist.Betty and Abby live relatively normal lives. They attend a regular school, and each does her own school work. They prefer to do some projects together, though. For example, to cut out paper dolls, one twin holds the paper, while the other uses the scissors. But sometimes the girls don’t want to do the same thing. For example, sometimes they want to play with different toys. What do they do then? "We toss a coin," says Abby.
Questions 16 to 18 are based on the passage you have just heard.
16. In what aspect do Betty and Abby differ from most twins?
17. What does the passage tell us about twins who share parts of the same body?
18. What does the passage say about the education of the twin girls?