input: Write a function that takes a sentence as input and returns a list of its verbs. The function should be named extract_verbs and should use the nltk library to process the sentence and identify the verbs.
  assert extract_verbs("The cat jumped over the moon.")==['jumped']
  assert extract_verbs("She sings beautifully.")==['sings']
  assert extract_verbs("He quickly ran to the store.")==['ran']
  assert extract_verbs("The dog barks loudly.")==['barks']
  assert extract_verbs("They dance on the stage.")==['dance']
output: import nltk
  from nltk.tokenize import word_tokenize
  from nltk.tag import pos_tag
  
  def extract_verbs(sentence):
      words = word_tokenize(sentence)
      tagged_words = pos_tag(words)
      verbs = [word for word, pos in tagged_words if pos.startswith('VB')]
      return verbs