input: Write a function to take a sentence as input and return a list of verbs extracted from the sentence. The function should use NLTK library for tokenization and part-of-speech tagging. If the input sentence is empty, the function should return an empty list.
  assert extract_verbs("The cat jumped over the moon.")==['jumped']
  assert extract_verbs("She is running fast.")==['is', 'running']
  assert extract_verbs("They went to the store.")==['went']
  assert extract_verbs("")==[]
output: def extract_verbs(sentence):
      if not sentence:
          return []
      from nltk import word_tokenize, pos_tag
      tokens = word_tokenize(sentence)
      tagged = pos_tag(tokens)
      verbs = [word for word, pos in tagged if pos.startswith('VB')]
      return verbs