input: Write a function to find the most common key in a dictionary using the Counter class from the collections module.
  assert most_common_key({'python': 20, 'java': 10, 'c++': 15, 'C#': 15}) == 'python'
  assert most_common_key({'a': 1, 'b': 1, 'c': 2}) == 'c'
  assert most_common_key({'x': 5, 'y': 5, 'z': 6}) == 'z'
output: from collections import Counter
  
  def most_common_key(d):
      counter = Counter(d)
      most_common = counter.most_common(1)
      return most_common[0][0]