One grade 12 class, each class has 50 students, and the random number is 1 ~ 50. In order to understand their interest in extracurricular activities, the 40th student in each class is required.

Dividing the population into balanced parts refers to dividing the population,

In paragraph 1, a starting number is determined by simple random sampling.

On the basis of this number, plus the integral multiple of the segmentation interval, it is the sampling number.

So choose D.