题目内容：
The last two sentences of the first paragraph mean that__________
A．a reading speed too slow for a difficult book is just right for a non-serious one B．a reading speed too slow for a non-serious book may be too fast for a difficult one
C．A reading speed too fast for difficult material is just right for a non-serious book is also too slow for a difficult one
D．A reading speed too slow for a non-serious book is also too slow for a difficult one
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