Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Driver | Constructor | Laps | Time/Retired | Grid |
| --- | --- | --- | --- | --- |
| Niki Lauda | Ferrari | 40.0 | 1:45:16.78 | 2.0 |
| Patrick Depailler | Tyrrell - Ford | 40.0 | + 21.47 | 9.0 |
| Tom Pryce | Shadow - Ford | 40.0 | + 23.84 | 12.0 |
| Hans-Joachim Stuck | March - Ford | 40.0 | + 1:28.17 | 14.0 |
| Jody Scheckter | Tyrrell - Ford | 40.0 | + 1:56.46 | 13.0 |
| Jochen Mass | McLaren - Ford | 40.0 | + 1:58.27 | 6.0 |
| Clay Regazzoni | Ferrari | 40.0 | + 2:15.24 | 4.0 |
| Jacky Ickx | Wolf-Williams - Ford | 39.0 | + 1 lap | 19.0 |
| Renzo Zorzi | Williams - Ford | 39.0 | + 1 lap | 17.0 |
| Carlos Pace | Brabham - Alfa Romeo | 39.0 | + 1 Lap | 10.0 |
| Ingo Hoffmann | Fittipaldi - Ford | 39.0 | + 1 Lap | 20.0 |
| Carlos Reutemann | Brabham - Alfa Romeo | 37.0 | Out of Fuel | 15.0 |
| Emerson Fittipaldi | Fittipaldi - Ford | 37.0 | + 3 Laps | 5.0 |
| Lella Lombardi | March - Ford | 36.0 | + 4 Laps | 22.0 |
| Jean-Pierre Jarier | Shadow - Ford | 33.0 | Accident | 3.0 |
| James Hunt | McLaren - Ford | 32.0 | Accident | 1.0 |
| Vittorio Brambilla | March - Ford | 15.0 | Oil Leak | 7.0 |
| Jacques Laffite | Ligier - Matra | 14.0 | Transmission | 11.0 |
| Ronnie Peterson | Lotus - Ford | 10.0 | Accident | 18.0 |
| Mario Andretti | Lotus - Ford | 6.0 | Accident | 16.0 |
| John Watson | Penske - Ford | 2.0 | Fuel System | 8.0 |
| Ian Ashley | BRM | 2.0 | Oil Pump | 21.0 |
Question:
What is the average laps for the grid smaller than 21 for Renzo Zorzi?
Answer:
39.0