A statistical review of word types and frequencies in all of the novels in a library has found that the novels published prior to 1970 use far fewer slang words, measured as a percentage of the book's total words, than the novels published in 1970 or later. Evidently, whether or not they intended to do so, authors of novels published prior to 1970 used less slang than authors of later books.
Which of the following, if true, most seriously weakens this argument?
【选项】
The amount of slang used in non-fiction books in the statistical review is essentially constant across periods of history.
选项是否正确？