单项选择题

The final goal of designing a lens is______.

A．to trap the opponent's lenses
B．to focus light with lenses
C．to hand-make lenses at low cost
D．to reflect light by means of curved surfaces
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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
单项选择题
One of the features of REM sleep is that______.
A．there are large slow waves, though rapid for the first few minutes
B．you have the deepest sleep
C．there are no brain waves
D．the brain waves are a little fast and the brain becomes a little active
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