Passage three

Imaging 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, too, 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. Because she has three or four lungs which provide
plenty of oxygen for both twins. Most of their completely shared organs
lie 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 18-20 are based on the passage you have just heard.
18.In what aspect, do Betty and Abby differ from most twins?
19.What does the passage tell us about twins who share parts of the same
body?
20.What does the passage say about the education of the twin girls?