题干

下列关于“观察DNA和RNA在细胞中的分布”实验的说法,正确的是(    )

A:甲基绿和吡罗红对DNA和RNA的亲和力不同,实验中应分别加入甲基绿和吡罗红

B:盐酸能够改变细胞膜的通透性,加速染色剂进入细胞

C:该实验用口腔上皮细胞而不用叶肉细胞,是因为叶肉细胞不含RNA

D:盐酸有利于染色质中DNA与蛋白质分离,不利于DNA与染色剂结合

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B

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    Teaching second grade is always a challenge. Each student arrives at school with his own needs and difficulties. One year a student called Billy 1me with his behavior as well as his academic requirements. He struggled daily with his 2emotions and often became angry or violent. I knew that, to make 3progress,his emotions needed controlling.

    One 4I tried to help Billy was to have him come directly into the classroom when he arrived at school. Billy's mom would 5me to alert(警告) me to a particularly emotional morning at home. Then, I would focus on 6his anger and calming him down before the other students arrived.

    One week our class was studying 7.I thought one way to bring learning into the classroom was to bring my dog Rocky to school for the day.

    That day began as normal. I was preparing activities focused on dog themes 8I was told that Billy had a 9morning at home and I might need to get him 10. As I was talking to his mom,Billy 11into the classroom. To Billy's12, Rocky immediately ran up to his new13, wagging his tail and licking Billy's face with doggy affection. Billy couldn't 14Rocky's charm(魅力)and began laughing as his anger melted away.

    Throughout the day,Billy never left Rocky's side, feeding him, being gentle with him and even 15the other students while Rocky was sleeping.

    Billy was known for doing anything he could to avoid16, but on this day he found a good dog story, “ Clifford's Puppy Days”, and read it to Rocky. How 17I was at the sight of Billy reading happily! My little dog was able to 18Billy's day from one of anger and frustration to one of laughter, gentleness and 19.

    That day Rocky more than helped me with my20; he helped to change the life of a child!After that Billy's behavior definitely improved.