单项选择题
A．They connect invisible phenomena to those we are familiar with. B．Scientists could not experiment without them. C．They give the scientist a sense of security. D．They provide deeper insight into the workings of the human min
A．working in a bank B．printing a book C．attending a night school D．growing beans for sale
A．Large, but cheap, easy to use. B．Large, expensive, very popular. C．Small, cheap, easy to use. D．Large, expensive, hard to us