A、It was lovely. B、It was warm. C、It was cold. C根据回答“但是昨天很冷。”可知昨天天气很冷，所以应选C。

W: What would you like for breakfast?
M: Bread and milk.
Q: What are they talking about?

W: Lovely weather today, isn’t it?
M: Yes, but yesterday was very cold.
Q: What was the weather like yesterday?