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 491, the last three numbers of our telephone number. Now I will explain how the alarm works: 1. There is a power siren 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 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 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 down stairs 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, re-set 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 tamed on the light and 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 animals)  disappear behind the curtains. "Oh!" I said to myself. "That's our burglar." When the lizard moved, it started the siren. I guessed 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 . The purpose of the writer in writing the passage is to  _  .
tell us that the alarm system doesn't work make it clear that he wants to give away his alarm system show us how the alarm system works tell us an interesting experience