input: Write a function to find the most frequent word in a sentence.
  assert most_frequent_word("this is a test this is only a test") == "test"
  assert most_frequent_word("hello world hello") == "hello"
  assert most_frequent_word("one two three two three three") == "three"
output: def most_frequent_word(sentence):
      words = sentence.split()
      word_count = {}
      for word in words:
          word_count[word] = word_count.get(word, 0) + 1
      most_frequent = max(word_count, key=word_count.get)
      return most_frequent