**Core Concept**
Direct standardization is a statistical method used to compare mortality rates between two or more populations, taking into account the differences in demographic characteristics such as age and sex. This technique is particularly useful when comparing populations with varying age distributions.

**Why the Correct Answer is Right**
When comparing mortality rates between two countries, direct standardization is used to account for differences in age distributions. This is because age is a significant risk factor for mortality, and populations with different age structures may have varying mortality rates due to this factor alone. By standardizing the age distribution, direct standardization allows for a more accurate comparison of mortality rates between the two populations.

**Why Each Wrong Option is Incorrect**
**Option A:** Cause of death is not the primary reason for using direct standardization, although it may be a consideration in certain analyses. Cause of death is more relevant to indirect standardization.
**Option B:** Numerators refer to the number of deaths in each population, which is not the primary reason for using direct standardization.
**Option D:** Denominators refer to the total population at risk, which is also not the primary reason for using direct standardization.

**Clinical Pearl / High-Yield Fact**
When comparing mortality rates between populations, it's essential to consider the age distribution of each population to ensure accurate comparisons. Direct standardization is a valuable tool in epidemiology for controlling for this confounding variable.

**✓ Correct Answer: C. Age distributions**