到底是 on holiday 还是on holidays？其中的 holiday 到底该用单数还得复数？
_________ the railways put on special trains.
A. In holiday B. On holiday C. In holidays D. On holidays
现分析如下：
大家知道，on holiday是习语，意为“在休假”，该短语通常用介词 on 而不用 in，且其中的holiday 通常不用复数，且其中也不用冠词，但有时可用物主代词 (此时可用复数，即 on one's holidays)。基于此认识，许多同学便选了B。
其实此题最佳答案为D，on holiday 和 on holidays 均可用说，只是含义不同：
1. 短语 on holiday，可用作表语或状语，其中通常不用冠词，但有时可用物主代词。如：
I’ll be on holiday next month. 我下个月全月休假。
The typist is away on holiday this week. 打字员本周休假。
I met a lot of interesting people on holiday. 我度假时碰到很多有趣的人。
We’re touring
They are on their holiday in
It’s costing me a small fortune to send children on holiday. 送孩子们度假花了我一大笔钱。
Where to go on holiday is always a bone of contention in our family. 到哪去度假总是引起家里的争执。
We are short of funds at the moment, so we are not going on holiday. 眼下我们缺钱，所以不去度假了。
We couldn’t decide where to go on holiday so we just picked a place out of a hat. 我们定不下来到哪儿去度假，就随便选了个地方。
We’re going on holiday with Mary and Daniel Sinclair. 我们要去与玛丽和丹尼尔•辛克莱一起度假。
It’s rather tough on him falling ill just as he’s about to go on holiday. 他正要去度假却生病了，真倒霉。
‘Queen Mother goes on
If you save some money each week, we can go on holiday. 要是你每星期能存点儿钱，咱们就可以外出度假了。
We always seem to take everything but the kitchen sink when we go on holiday. 我们度假时好像总把一切能带的东西都带着。
They always take everything but the kitchen sink when they go on holiday. 外出度假时，他们除了厨房水槽，把什么都带上了。
Our team dropped behind in the league matches when a third of the players went on holiday. 我们的队伍由于三分之一的队员在度假而在联赛中落后了。
2. 短语 on holidays，主要是用来表示诸如元旦、国庆等假日。如：
Travel is very heavy here on holidays. 假日这里的交通很拥挤。
On holidays the railways put on special trains. 假日铁路增开专列。