input: Write a python function to find the word that appears the most frequently in the provided list of strings.
  assert most_frequent_word(["UTS is best for RTF", "RTF love UTS", "UTS is best"] ) == 'UTS'
  assert most_frequent_word(["Its been a great year", "this year is so worse", "this year is okay"] ) == 'year'
  assert most_frequent_word(["Families can be reunited", "people can be reunited", "Tasks can be achieved "] ) == 'can'
output: def most_frequent_word(sentences):
      from collections import Counter
      words = []
      for sentence in sentences:
          words.extend(sentence.strip().split())
      word_count = Counter(words)
      return max(word_count.keys(), key=(lambda key: word_count[key]))