input: Write a function to divide two numbers and then divide the result by another two numbers with optional parameters.
  assert divide(8, 2) == 4.0
  assert divide(8, 2, 2, 1) == 2.0
  assert divide(12, 3, 4, 2) == 2.0
output: def divide(a, b, c=1, d=1):
      first_division = a / b
      second_division = c / d
      return first_division / second_division