最佳日接待量制定的主要依据是（   ）

Different countries have different food, such as French fries, Italian pizza and so on. In China, fast food is becoming more and more popular these days. For example, McDonald's and KFC. Why do Chinese people like fast food so much? There are many reasons. First, it is quick. It takes little time. Second, it is easy to carry. Third, people like the restaurant's relaxing environment and American style. Though it tastes good, fast food generally contains little nutrition (营养). If you want to have a healthy body, do not go to fast food restaurants too often. Eat more fruit and vegetables.

扦插时，保留有芽的插枝比较容易生根成活，这是因为（　　）