input: Write a python function to find the most common string in a list.
  assert most_common_string(['a', 'b', 'a', 'c', 'a', 'b']) == 'a'
  assert most_common_string(['x', 'y', 'y', 'z', 'x', 'x', 'y']) == 'y'
  assert most_common_string(['apple', 'banana', 'apple', 'orange', 'banana', 'apple']) == 'apple'
output: def most_common_string(strings):
      from collections import Counter
      count = Counter(strings)
      return count.most_common(1)[0][0]