单项选择题

Good locks on all outside and first floor windows are essential to a home’s security and will go a long way toward discouraging burglars. But an added protective device that is increasingly being used in homes is the burglar system, which sounds a loud bell, horn, or siren (警报器) when someone tries to break in. Although this will not always prevent entry by a determined professional burglar, it is often enough to scare off vandals (故意破坏他人财物者) and spurn-of-the- moment thieves who account for a high percentage of residential break-ins.
Security alarm systems have been available for many years and have long been common in commercial and industrial establishments and in large homes. But until the last few years, these systems were almost always professionally installed because they required complicated wiring techniques, individualized design, and sometimes lots of carpentry, all of which made them expensive.
To answer the demand for low-cost alarm systems that can be easily installed, several companies now make alarms that are specifically designed for do-it-yourselfers. These systems come with detailed installation instructions and usually do not require inside-the-wall wiring.
 

In the past, people did not have security alarm systems installed in their homes, because（）

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