单项选择题

After the passage the author will talk about______.

A．the principles of designing lenses
B．techniques of making contact lenses
C．the design of photographic lenses
D．styles of lenses
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单项选择题
The brain waves are the slowest during______.
A．stage 1
B．stage 2 and stage 3
C．stage 4
D．REM sleep
单项选择题
The stages of sleep take on______.
A．an irregular aspect.
B．a regular aspect
C．a punctual aspect
D．a similar aspect
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