限制：数据范围不能太大，否则空间浪费
The “Knocking on Wood” Wikipedia page mentions Roud’s conclusion, and this claim of a 19th century origin has circulated online as the final say on the matter. If you go back to the original, though, Roud’s proposal actually ends on an ambivalent note: “Before this theory [of 19th century origins] can be finally accepted, however, an examination of the history of European forms of this custom would be advisable.”，推荐阅读搜狗输入法2026获取更多信息
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美国：企业自救，市场化封闭生态
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