题干

家养禽畜和栽培作物比在自然状态下显示出丰富的多样性,这主要是长期受人工选择的影响.下列关于人工选择的叙述,不正确的是( )

A:人们对生物的性状选择有多方面的需要

B:人工方法能获得更多的变异来源

C:人们通过生产活动能创造更丰富的环境条件,使对人类有利的变异得到保存

D:人工选择可不受自然条件的制约

上一题 下一题 0.0难度 选择题 更新时间:2018-07-16 04:59:52

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D

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    In high school, all Canadian students take part in a very exciting event. It is "Take Your Children to Work Day". Students will have a chance to work with their parents. My day took place in my mom's work, the York Region Police Department.

    When I arrived, a coach was teaching the officers basic combat(格斗)skills. Everyone tried their best to practice. I could see them sweating and panting (喘气).

    The most exciting part of my visit was a stop at the 911 Communication Center. 911 is the same as 110,119 and 120 in China. Every 911 call in the city is answered here. When I entered the center, I stood beside a call dispatcher (分派人员)arid watched him dealing-with the calls.

    Suddenly, the phone rang, he picked it up and a woman cried for help. There was a robbery in her house. The call dispatcher told her, "Please calm down and move to the safe place quietly. Don't fight with them. Just let them take what they want. Please make sure you are not hurt. We are sending officers there."

    He followed the call and found the address. The call dispatcher chose the five closest police cars from the satellite map and clicked(敲键盘) to send the policemen.  I saw the police cars from the satellite map were quickly moving to the woman's house. Five minutes later, they arrived .I asked the call dispatcher why he sent five cars. He said, "There are different levels of emergencies. Robbery is a high priority (优先),so we have to send five cars." The day was interesting. I leaned how the different departments worked together. The officers were very friendly and admirable.