关于x、y的方程组$\left\{\begin{array}{c}x+m=6\\ y-3=m\end{array}\right.$中，x＋y＝____ 

种群在理想环境中呈“J”型曲线增长(如曲线甲)，在有环境阻力条件下，呈“S”型曲线增长(如曲线乙)。下列有关种群增长曲线的叙述，正确的是（    ）

声音与人的生活息息相关，为了认识声音，某实践活动小组设计了以下实验对声音进行了探究．

实验一：使正在发声的音叉接触水面溅起水花．

实验二：用细线把一个小铃铛悬挂在软木塞的下端，并置入烧瓶中，摇一摇瓶子，会听到清晰的“叮当”声．然后取下软木塞点燃酒精棉球放入瓶中，待火熄灭后，迅速塞紧软木塞，再摇瓶子，听到瓶里的响声比原来小很多．

实验三：支起自行车，一手转动自行车的脚踏板，另一只手则拿一硬纸片并让纸片一头伸进自行车后轮的辐条中，随着车轮转速的加快，纸片发出的声音会越来越尖．

（1）以上三个实验中，实验 ____表明发声体在振动．

（2）实验二表明： ____；

（3）实验三表明： ____

A

    Mrs. Gill lived next door to Nita. Every afternoon, Mrs. Gill took a walk. She said it kept her young. Nita liked to talk with Mrs. Gill when Mrs. Gill came back. Mrs. Gill was always happy after her walk. She often showed Nita something she had found, like a beautiful red leaf (叶子)

    One afternoon, Mrs. Gill looked upset. Nita asked her why.

    “I am sad about all the trash (垃圾) on the street,” she said. “I never saw so much trash in this neighborhood! What happened?”

    “Mrs. Trent moved,” said Nita. “She used to go out each morning and pick up trash. I often saw her on my way to school.”

    Mrs. Gill looked very    ▲   . “I thought I knew everything about this neighborhood!” she said. “But you see more than I do!”

    “Well, I wish I could do Mrs.Trent's job. But you know, my back (背) often hurts. It's really hard for me to pick things up,” she added.

    “I could do that,” said Nita.

    Mrs. Gill said, “I will carry the trash bag! You can use my gloves (手套) so your hands can stay clean.”

    Nita put on the gloves. She picked up pieces of paper. She picked up boxes. And Mrs. Gill carried the bag. Lots of people stopped to thank Mrs. Gill and Nita. The street looked much better.

    “Thanks, Nita,” said Mrs. Gill. “Let's call ourselves the clean-up team!”

根据材料内容选择最佳答案。