| List I (Concept) | List II (Definition) | ||
| A. | Density of population | I. | The working population participation in various occupations |
| B. | Sex Ratio | II. | Numbers of people in different age groups |
| C. | Age structure | III. | Ratio between the number of women and men |
| D. | Occupational Structure | IV. | Ratio between the numbers of people to size of land |
What comes next in the series?
\(2, 6, 12, 20, 30, \ ?\)