单项选择题
A. The kinds of molecules in which carbon may be replaced by silicon. B. The importance of silicon. C. The difference between molecules and silicon.
A. It allowed workers to spend more time at home. B. It gave workers opportunity for better training. C. It let workers live far from their jobs.
A. How to teach arts and crafts to children and adults. B. How to perform in front of a large audience. C. How to use sign language in the theater.