input: Write a python function to return the English word for a given digit (0-9).
  assert digit_to_word(0) == "This is a zero."
  assert digit_to_word(3) == "This is a three."
  assert digit_to_word(7) == "This is a seven."
  assert digit_to_word(9) == "This is a nine."
output: def digit_to_word(digit):
      words = ["zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine"]
      return f"This is a {words[digit]}."