input: Write a function to return the commission rate based on the amount of gross sales. The commission rates are 0.1 for gross sales greater than or equal to 10000, 0.07 for gross sales between 5000 and 10000, and 0.05 for gross sales less than 5000. If the gross sales is not a positive number, return None.
  assert determine_commission_rate(12000)==0.1
  assert determine_commission_rate(7500)==0.07
  assert determine_commission_rate(3000)==0.05
  assert determine_commission_rate(-500)==None
  assert determine_commission_rate(10000)==0.1
output: def determine_commission_rate(gross_sales):
      if gross_sales < 0:
          return None
      elif gross_sales >= 10000:
          return 0.1
      elif gross_sales >= 5000:
          return 0.07
      else:
          return 0.05