Number (0): Everything in this space must add up to 0. The answer is 3-0, placed vertically.
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.”，详情可参考heLLoword翻译官方下载
。旺商聊官方下载是该领域的重要参考
Фото: Dmytro Buianskyi / Shutterstock / Fotodom。关于这个话题，im钱包官方下载提供了深入分析
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