The death rate among army personnel is $8$ per $1000$, while the death rate among civilians is $20$ per $1000$. The comparison is made between two distinct groups: army personnel and civilians. These two groups are heterogeneous in nature, meaning they have different characteristics, environments, and lifestyles. Since the groups are different in many ways, directly comparing the death rates can be misleading. For example, army personnel may have better access to healthcare, are generally healthier, and have a more controlled environment compared to civilians. So, the comparison is not valid without considering these differences.

Therefore, the correct inference is: D. The groups cannot be compared due to their heterogeneity.