Tipping is neither encouraged nor mon, although visitors may reward special services if they wish, 10% of the bill is enough.
A which B where C in which D in which case
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解析：这题考查 定语从句的：介词+which+名词的用法，句意是：虽然游客如果愿意可以奖励特殊的服务，但是给小费不被鼓励也不常见，这种情况下，账单的10%足够了。选D。D。