input: Write a function to determine the commission rate based on the gross sales. The commission rate is 10% if the gross sales are greater than or equal to 10000, 7% if the gross sales are between 5000 and 9999 (inclusive), and 5% if the gross sales are less than 5000. The function should return the commission rate as a floating-point number. If the gross sales are less than 0, the function should return 0.0.
  assert determine_commission_rate(12000)==0.1
  assert determine_commission_rate(7500)==0.07
  assert determine_commission_rate(4000)==0.05
  assert determine_commission_rate(-500)==0.0
  assert determine_commission_rate(0)==0.05
output: def determine_commission_rate(gross_sales):
      if gross_sales < 0:
          return 0.0
      elif gross_sales >= 10000:
          return 0.1
      elif gross_sales >= 5000:
          return 0.07
      else:
          return 0.05