input: Write a python function to extract keys from a dictionary where all values are 'fruit'.
  assert extract_fruits({'apple':'fruit', 'carrot':'vegetable', 'banana':'fruit'}) == ['apple', 'banana']
  assert extract_fruits({'peach':'fruit', 'cucumber':'vegetable', 'plum':'fruit', 'tomato':'vegetable'}) == ['peach', 'plum']
  assert extract_fruits({'apple':'fruit', 'orange':'fruit', 'mango':'fruit'}) == ['apple', 'orange', 'mango']
output: def extract_fruits(d):
      return [key for key, value in d.items() if value == 'fruit']