形 one，single，odd，unlined；副 singly，alone，only，solely；名 sheet，bill，list；

[例句] 侍者把他们点的餐端了过来，格雷厄姆签了单。 The waiter returned with their order and Graham signed the bill.

[例句] 为什么我们要为酒吧和宾馆不必要的暖气开支买单呢？ Why do we pay to overheat pubs and hotels?