听一段材料，回答下面几个小题。
What are they talking about?
A.A story about a friend who was on a sweet potato diet.
B.How to keep diet.
C.People should eat all kinds of things.
Why was the friend ill?
A.Because he wanted to lose weight.
B.Because sweet potatoes can’t provide him with everything he needs.
C.Because he ate sweet potatoes.
What is the best way to keep diet?
A.To eat every kind of food.
B.To eat less meat.
C.To eat less and exercise more.