input: Write a python function to find the maximum value in a dictionary and return its corresponding key.
  assert max_key({"A": 4, "B": 6, "C": 10}) == "C"
  assert max_key({"X": 2, "Y": 8, "Z": 5}) == "Y"
  assert max_key({"P": 12, "Q": 7, "R": 12}) == "P"
output: def max_key(d):
      return max(d, key=d.get)