input: Write a function to extract the verbs from a given sentence using the NLTK library. If the input sentence is empty, return an empty list.
  assert extract_verbs("The cat jumped over the moon.")==['jumped']
  assert extract_verbs("She is running quickly.")==['is', 'running']
  assert extract_verbs("He eats an apple.")==['eats']
  assert extract_verbs("They walked to the park.")==['walked']
  assert extract_verbs("")==[]
output: import nltk
  nltk.download('punkt')
  nltk.download('averaged_perceptron_tagger')
  
  def extract_verbs(sentence):
      if not sentence:
          return []
      words = nltk.word_tokenize(sentence)
      tagged = nltk.pos_tag(words)
      verbs = [word for word, pos in tagged if pos.startswith('VB')]
      return verbs