input: Write a python function to extract verbs from a given sentence using NLTK library.
  assert extract_verbs("The cat jumped over the moon.") == ['jumped']
  assert extract_verbs("He runs quickly and jumps high.") == ['runs', 'jumps']
  assert extract_verbs("She is singing a beautiful song.") == ['is', 'singing']
output: def extract_verbs(sentence):
      import nltk
      from nltk import pos_tag, word_tokenize
      nltk.download('punkt')
      nltk.download('averaged_perceptron_tagger')
      
      words = word_tokenize(sentence)
      pos_tags = pos_tag(words)
      
      verbs = [word for word, pos in pos_tags if pos.startswith('VB')]
      return verbs