input: Write a function to extract all the verbs from a given sentence. The function should take a sentence as input and return a list of verbs.
  
  import nltk
  
  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("Walking is good for health.") == ["Walking", "is"]
  assert extract_verbs("") == []
output: def extract_verbs(sentence):
      tokens = nltk.word_tokenize(sentence)
      pos_tags = nltk.pos_tag(tokens)
      verbs = [word for word, pos in pos_tags if pos.startswith('VB')]
      return verbs