Suppose that Italy and Switzerland both produce fish and olives. Italy's opportunity cost of producing a crate of olives is 3 pounds of fish while Switzerland's opportunity cost of producing a crate of olives is 10 pounds of fish.

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Doulingo is good. you can also watch movies that are in french, i am using this tactic  for japanese  and it a lot. listing to music with  french  lyrics would work well too because  you could absorb  the words while having fun.hope this ! : )

Harmonia by rythemyoure welcome