单项选择题


Questions 23 to 25 are based on the conversation you have just heard.

A．Planktons are too small to be seen.
B．Most planktons have transparent tissues.
C．Most planktons are practically invisible to predators.
D．Planktons are fascinating organisms.
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Language problems may come to the attention of the public when they have ______ such as fatal accidents or social problems.
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