Final answer:
The fact that half of the dorm residents still got colds despite 80% of them taking vitamin C last winter suggests that vitamin C is not a guaranteed prevention.
Step-by-step explanation:
The fact that half of the dorm residents still got colds despite 80% of them taking vitamin C last winter suggests that vitamin C is not a guaranteed prevention. While it may reduce the duration, severity, and incidence of the common cold, it is not 100% effective in preventing it. Other factors such as dorm living conditions, exposure to viruses, and individual immune systems may also contribute to the occurrence of colds.