Which of the following groups are the most vulnerable to dehydration

The answer is : Infants and elderly

The major cause of this vulnerability is most of the people in this groups could not provide their need on their own. Not only that, they also could not communicate well to ask others for helps to fulfill their need ( infants could only cried and a large numbers of elderly lost their ability to speak properly)

Ithink it's a, but i might be wrong