问题 选择题
设A、B两镇相距x千米,甲从A镇、乙从B镇同时出发,相向而行,甲、乙行驶的速度分别为u千米/小时、v千米/小时,并有:
①出发后30分钟相遇;
②甲到B镇后立即返回,追上乙时又经过了30分钟;
③当甲追上乙时他俩离A镇还有4千米.求x、u、v.
根据题意,由条件③,有四位同学各得到第3个方程如下,其中错误的一个是(  )
A.x=u+4B.x=v+4C.2x-u=4D.x-v=4
答案

答案:A

单项选择题 A1型题
单项选择题

Questions 1-5Last month, upon hearing that a neighbor had been burgled, my husband voiced a desire to beef up our home security. I was largely unresponsive. The previous owners of our house installed a burglar alarm system, but we never got it switched on, because, quoting Ed, I apparently care more about the $29 monthly fee than I do about our home security. In the end, I gave in.The alarm company sent over a sales representative, a well-coiffed professional in a suit and heels. She recommended adding some infrared motion sensors. I was not wild about this. I like to keep things simple. My idea of home security is to hire cheap, disreputable painters who can be counted upon to paint the windows shut. "Besides, can’t the motion sensors be set off by a pet " I said.Ed leaned in close to the sales rep. "We don’t have any pets," he whispered. "We don’t have a pet now’’ I said." But we might someday." I knew this to be a lie. Ed is a dog person, and I’m a cat person. We cancel each other out.I pointed out that every now and then, the neighbors’ cat, Sprinkles, will sneak into the house when the back door is open. The alarm woman started talking about "pet resistance." This was a feature of the motion sensor whereby it was set to cover the room from the waist up only. "Though of course...," she hesitated, "the cat would have to stay on the ground at all times."We got the sensors, and we got the system switched on. We never got a pet, each of us practicing his or her own particular brand of pet resistance, but we did, after many years of cost-based bickering, get a housecleaner. Every other month, Natalia can be seen making her way through the filth and cobwebs. I gave her the alarm code but promised to leave the alarm off the day she came.Naturally, I forgot. Later that morning, my work phone rang. It was Natalia, yelling in harmony with the shrieking of the alarm. She couldn’t find the code. On top of all this, my cell phone started ringing. This was the alarm company, responding to the alarm and calling me to get the secret password-which was different from the shutoff code-required for them to shut off the system and prevent the police from rushing over to arrest Natalia for breaking and entering.Some weeks back, Ed and I had spent 15 minutes arguing over the secret password for the alarm. Ed is a fan of the complicated, hacker-proof, identity-theft-foiling password, the kind that involves alternating capital and lowercase letters with obscure foreign accent marks, whereas I’ll use my name. I had no recollection of what we’d settled on. "Ummmm." The alarm, and Natalia, continued to go off. This went on for some time.Meanwhile, Natalia had dug through her bag, found the piece of paper I’d given her with the shutoff code and quieted the screaming alarm. I don’t know how effective these alarms are against burglars, but Sprinkles hasn’t been seen on the property in weeks.

According to the sales representative, the motion sensor _______.

A.is pet resistant

B.is set to cover the room floor

C.could be set off by a pet if it was near

D.could be set off by a pet if it jumped high enough