Is there a grammar rule that can help me know which preposition is correct to use for periods of time such as this? My source of confusion comes from another exercise in which "nella mattina" was incorrect (the correct choices were "I gatti bevono latte ALLA mattina, or ...DI mattina). So in this exercise, why is it "NEL pomeriggio" but in a similar context I cannot use NELLA mattina? In this question, can we also say "AL pomeriggio" or "DI pomeriggio"? Thanks in advance!