单项选择题

It’s your worst nightmare: you arrive back home after work or a holiday to find that someone else has moved into your fiat or house. They play their music loudly on your recorder, sleep in your bed and make a terrible mess (脏乱). And they refuse to leave. This is what many people understand by the term "squatting".
Since the 1970s, squatting has become relatively common in Britain. People (most of them young) who can’t afford to rent a place to live in, occupy a building left empty by a private landlord or the local housing authority. At present, squatting in empty properties is illegal but it’s not crime. Getting a squatter out of a property is a long procedure involving a court action. And while this is happening, the owner is not allowed to force his way into his own property.
Now the British government is hoping to introduce a law which would make life more difficult for squatters. The proposed new law would allow courts to order squatters to leave a property within 24 hours. If they didn’t they could be given a six-month prison sentence (and therefore a criminal record) or a fine of up to 5,000 pounds. It would also give police or the representatives of the courts—the bailiffs (法警)—more powers to break into property to force squatters to leave.

The word "nightmare" in this passage (Line 1, Paragraph 1) means（）

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