Answer/Explanation:
A family has a farm in which they grow crops and keep cows. During a particularly harsh winter, the crops failed to grow as well as they normally do. This means that their food supply was limited.
However, they also have cows, and cows produce milk. If this family were able to drink milk, they could get some nutrition from the milk. This means they are less likely to succumb to malnutrition and disease, and are more likely to survive and reproduce.