The continuous smog weather this year has brought great influence to people's lives, and people often hear concepts such as "dust particles" and "PM2.5".

For the evenly distributed and representative respondents, the five-point sampling method can be adopted: firstly, the midpoint of the diagonal line is determined as the central sampling point, and then four points on the diagonal line with the same distance from the central sampling point are selected as the sampling points. If it happens to be on the sideline, follow the principle of not being left or right.

So choose: C.