input: Write a function to extract phone numbers that start with '08' from a given list of phone numbers. Return a list containing only those phone numbers that match the criteria.
  assert extract_numbers(['0834567', '0945678', '07123456', '08901234', '07890123'])==['0834567', '08901234']
  assert extract_numbers(['0912345', '07654321', '08765432'])==['08765432']
  assert extract_numbers(['09876543', '07123456'])==[]
  assert extract_numbers([])==[]
output: def extract_numbers(phone_numbers):
      return [num for num in phone_numbers if num.startswith('08')]