以下细胞结构中，不含有脂类成分的是（　　）

如图，在△ABC中，AB＝AC＝4cm，∠BAC＝90°．动点P、Q同时从A、B两点出发，分别沿AB、BC方向匀速移动，它们的速度都是1cm/s，当点P到达点B时，P、Q两点停止运动．设点P的运动时间为ts，四边形APQC的面积为ycm²．

（1）当t为何值时，△PBQ是直角三角形?
（2）①求y与t的函数关系式，并写出t的取值范围；
②当t为何值时，y取得最小值？最小值为多少？
（3）设PQ的长为xcm，试求y与x的函数关系式．

如图为北半球近地面某气压场中的受力平衡的风向图，图中字母a代表的是（　　）

下列依次在横线处填入的词语和句子，语意和衔接都最恰当的一项是(    )
人的视觉接受神经系统对黑白特别____(1)____，这是因为构成人的视网膜边缘的一亿多个视杆细胞分辨黑白的本领特别强，即使____(2)____，也能____(3)____一个由明暗组成的黑白世界。可见，____(4)____，从而为黑白摄影被人的视觉接受及欣赏奠定了生理和心理基础。
①敏感
②敏锐
③在深夜感觉不到色彩
④在感觉不到色彩的深夜
⑤感受
⑥感知
⑦观众的视觉有优先唤起黑白的能力
⑧黑白有优先唤起观众视觉的能力

根据短文内容，从方框中选择恰当的单词填空。有的需要变换形式

communicate，wrong，offer, second，instead，opinion，any more，argue

    Sally Seaman is an American Writer. She tells young students some smart ways to make friends. In her ____, founding(建立) friendship is just like planting a tree. You should plant the seed(种子) and take care of it to make it grow.

    Firstly, you should choose a friend. A good friend should be kind and patient(有耐心的). For example, if something bad happens to you, a good friend could listen to you. And he or she should ____to help you. 

    ____, you can't be too shy. You should often ____with your friend. You should share your lives with your friend. 

    However, even best friends have fights. What should you do when you ____with your friend? You won't talk to him or her. That's ____. If so, you won't be friends ____. You need to keep on talking to him or her ____. If he or she doesn't want to talk, you can write a letter. That works.