We should try to help the ____________ because they have an illness which makes it difficult for them to move around.

阅读短文，从A、B、C、D四个选项中，选出可以填入空白处的正确答案。
    Look at this picture. It is 1 nice classroom. In the picture, you can see a teacher's 2 on his desk. It's a Chinese book. Eight desks and eight chairs in the room. You can see a girl and 3 boys.
    The girl's 4 is Julia. She is eleven. The boy 5 a black hat is Tom. The other boy is Tony. They 6 the same coats and they are the same age. I 7 they are twins. Tom is a 8 boy. A book is in his hand. You 9 see the teacher . 10 is the teacher? He is taking the photo

近年来，我国多个城市遭遇雾霾天气，空气中可吸入颗粒（又称PM2.5）浓度升高，为应对空气污染，小强家购买了空气净化器，该装置可随时显示室内PM2.5的浓度，并在PM2.5浓度超过正常值25（mg/m³）时吸收PM2.5以净化空气．随着空气变化的图象（如图），请根据图象，解答下列问题：

（1）写出题中的变量；

（2）写出点M的实际意义；

（3）求第1小时内，y与t的一次函数表达式；

（4）已知第5﹣6小时是小强妈妈做晚餐的时间，厨房内油烟导致PM2.5浓度升高．若该净化器吸收PM2.5的速度始终不变，则第6小时之后，预计经过多长时间室内PM2.5浓度可恢复正常？