单项选择题

Why is it hard to see most plankton

A．They are microscopic.
B．They move very quickly.
C．They are transparent.
D．They are rarely found near the surface.
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将before改为after。
单项选择题
The main idea of the passage is that______.
A．better use of green-space facilities should be made so as to improve the quality of our life
B．attention must be directed to the improvement of recreative possibilities
C．the urban environment is providing more recreative activities than it did many years ago
D．priority must be given to the development of obligatory activities
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