题目内容：

Last week, I bought an alarm system for about $ 450. It consisted of a control unit with three small units. I put the control unit in the sitting-room and fastened the other units by the front door, back door and living-room windows. The instructions told us to choose three numbers, so we chose 4, 9 and 1, the last three numbers of our telephone number.

Now I must explain how the alarm works:

l There is a power siren(警报器) in the control unit. It makes a very loud noise.

2 Each of the small units sends out beams or rays in different directions. If anything moves, it breaks a beam. This sends a signal to the control unit. The siren makes a noise which you can hear 50 meters away.

3. When we go to bed, I press the three buttons numbered 4, 9 and 1. Then we have 30 seconds to get out of the room before the alarm starts to work.

That night I slept soundly because I was sure that no burglar(夜贼) could get into our house. However, at about 2:10 a. m., I woke up and heard the siren. “There must be a burglar in the house,” Mary said. “What shall we do?”“I’ll go and see who’s there,” I said. “Stay here. Don’t make a noise.”I went downstairs quietly. When I reached the living-room, I switched on my torch and looked round the room. Then I turned the light on. I switched the siren off and searched the rooms downstairs. There was nothing wrong except that the back door was unlocked. I locked it, reset the alarm and went back to bed. About an hour later, the alarm started again. I jumped out of bed, fell over a chair in the dark and bumped into the bedroom door. Mary woke up and started hitting me with a torch. “Hey! Wait a minute!” I whispered. “It’s only me. I’m going downstairs to see what’s wrong.”I went down into the living-room and listened for a moment. The only sound I could hear was the siren. I turned on the light and then switched the siren off. As I did so, I glanced across at the curtains in front of the windows. I saw a house lizard (a kind of small animal) disappear behind the curtains.“Oh!” I said to myself. “That’s our burglar. ”When the lizard moved, it started the siren. I guessed that the alarm had been made in Europe, where there are no house lizards. Well. I won’t finish this story but if you want an alarm system free of charge, let me know. I’ll send you ours. We bought a dog this morning. It knows the difference between a lizard and a burglar.

1.The writer bought an alarm system to ________.

A．make sure that he got up early every day

B．warn him when there were lizards in his house

C．frighten burglars and tell him that somebody had got into the house

D．make Mary feel safe to live in the large room

2.The purpose of the three units was to________.

A．send a message to the control unit and start the siren

B．sound their own sirens when the control unit told them so

C．check that the control unit worked properly all the time

D．give them enough light

3.If the writer set the alarm and remained in the room for more than half a minute, _______.

A．nothing would happen

B．the control unit would not work

C．the siren would make a noise

D．the dog would bark

4.The purpose of the writer in writing the passage is to ________.

A．tell us that the alarm system doesn’t work

B．tell us an interesting experience

C．show us how the alarm system works

D．make it clear that he wants to give away his alarm system