Population disparity drives group differences

Steps:

• Identify the groups: injecting drug users (higher risk) and non-injecting drug users (lower risk).
• Examine the difference: likely higher HIV prevalence or cases in injecting users.
• Consider factors: transmission risks like needle sharing versus overall numbers.
• Conclude cause: uneven population sizes amplify observed disparities.

Why C is correct:

• Larger injecting drug user population increases total cases or rates, per epidemiological principles of prevalence scaling with group size.

Why the others are wrong:

• A: Frequency of use doesn't explain inter-group differences in outcomes like infection rates.
• B: Needle sharing raises risk but doesn't account for baseline numerical disparity between groups.
• D: Population density affects general HIV spread but ignores specific drug user group sizes.

Final answer: C