Answer:
C
Step-by-step explanation:
A. The weather was so hot that many people wore shorts every day.
^ this is clearly a fact. the person is observing that, for multiple days, people wore shorts because of the heat.
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B. My brother and I went to three different water parks this summer.
^ this is clearly a fact. the person is saying that they and their brother went to multiple water parks over the summer season.
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C. We will remember this as the best summer of our lives.
^ this is an opinion. someone else might think it was horrible, or just like any other summer. someone else may not like water parks at all, and to them this would have been a terrible, or rather an ''eh'' summer.
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D. Dad saved up enough money to buy a boat for the lake.
^ this is clearly a fact. the person is saying that their dad had been saving up money, and finally had saved up enough to buy a boat that summer.