If 28 percent of a country's population is aged 0-14 and the total dependency ratio is 75, why can the old-age dependency ratio not be calculated directly from this information alone?

Correct answer: Because the working-age percentage is also required

The total dependency ratio usually compares all dependents—children and older people—with the working-age population. The old-age dependency ratio, however, uses only older people in the numerator and the working-age population in the denominator. Knowing that children form 28 percent of the total population does not reveal how many older people there are or, by itself, the exact size of the working-age group.

The total ratio of 75 combines child and old-age dependency. To separate the older component, the working-age population and either the child-dependent or elderly count must be known in compatible terms. Thus option A is the best answer. The other statements are false: total dependency is meaningful, child share can be useful, and old-age dependency is not determined only by birth rate.