题目内容：
根据下列材料，请回答题A study of more than five million books, both fiction and non-fiction, has found a marked decline in the use of emotional words over time. The researchers form the University of Bristol used Google Ngram Viewer, a facility for finding the frequency of terms in scanned books, to search for more than 600 particular words identified as representing anger, dislike, fear, joy, sadness and surprise.
They found that almost all of the categories (类别) showed a drop in these “mood words” over time. Only in the category of fear was there an increase in usage.
“It is a steady and continuous decrease,” said Dr Alberto Acerbi. He assumed that the result might be explained by a change in the position occupied by literature, in a crowded media landscape. “One thing could be that in parallel to books the 20th century saw the start of other media. Maybe these media—movies, radio, drama—had more emotional content than books.”
Although both joy and sadness followed the general downwards trend, the research, published in the journal PLOS One, found that they also exhibited another interesting behaviour：the ratio (比率) between the two varied greatly, apparently mirroring historical events.
During the Roaring Twenties the joy-to-sadness ratio reached a peak that would not occur again until before the recent financial crash. But the ratio plunged at the height of the Second World War. Nevertheless, the researchers held a reserved opinion about their claim that their result reflected wider social trends. In the paper, they even argue that the reverse could be true.
“It has been suggested, for example, that it was the suppression (压抑) of desire in ordinary Elizabethan English life that increased demand for writing ‘filled with romance and sex’… perhaps,” they conclude, “songs and books may not reflect the real population any more than catwalk models reflect the average body.”
(Note：Answer the questions or complete the statements in NO MORE THAN TEN WORDS.)
A study of more than five million books indicated a decline in “mood words” over time except_______________.
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