The last two sentences of the first paragraph mean that______.A．a reading speed too slow f

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

请帮忙给出正确答案和分析，谢谢！