Answer:
We know that Dan wants collect data about eating habits of his friends, that means all quesiton must be towards that aim.
First, "What time of the day do my friends eat their biggest meal?" is a mandatory question he must include because it's including variables he wants to measure, eating habits, the time when they have the biggest meal sets a habit. If the take in the biggest meal at dinner, lunch or breakfast.
Second, "How many times each week do my friends eat out?" is also inside the purpose of Dan's study, beacuse it's about eating habits, maybe they go out for food everyday, or twice a week, those are habits.
Finally, "What types of foods do my friends prefer?" this is a important question Dan must included, because knowing the type of food determine if they have healthy eating habits or not.
In general, notice that these three questions offer enough information to describe the eating habits of his friends.
