B says "the third visit", maybe it's all happening in one visit. I think C and D are correct.

C is the only correct answer. D implies if you buy two pizzas, you get one of the two for free. It doesn’t explicitly say that the third pizza is the free one if you buy two.

It looks like they are all true, except for B. Maybe they made a mistake in the book.

Often in a grocery store or in a restaurant you well see one of these statements: A) Buy two, get one free! B) Three for the price of two! Suppose one pizza normally costs $6. The meaning of A and B are crystal clear if you want to buy 3 pizzas. However the meaning of B is not clear if you want to buy a different number of pizzas. In either case, the cost of 3 pizzas will be $12. That is indisputable. Things become more interesting when you want to know what will be the price of just one pizza. In the restaurant that advertises A, the answer is clear. One pizza will still cost $6. Two pizzas will cost $12. Three pizzas will cost $12. Thus the price of two pizzas will be the same as the price of three pizzas and both C and D are correct answers. However! And, this is where things start to get interesting. In the restaurant that advertises B, the price of just one pizza is not at all clear. You need to ask the restaurant employee if you want to know the answer. Since three pizzas cost $12, it is reasonable for the cost of one pizza to be $4. However, you cannot be certain of that because the advertisement only gives you information about three pizzas. It does not tell you anything about the price of just one or just two.

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