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 |
| --- | --- | --- | --- | --- |
| Gerhard Berger | Benetton - BMW | 68.0 | 1:33:18.700 | 4.0 |
| Alain Prost | McLaren - TAG | 68.0 | + 25.438 | 6.0 |
| Ayrton Senna | Lotus - Renault | 68.0 | + 52.513 | 1.0 |
| Nelson Piquet | Williams - Honda | 67.0 | + 1 Lap | 2.0 |
| Nigel Mansell | Williams - Honda | 67.0 | + 1 Lap | 3.0 |
| Philippe Alliot | Ligier - Renault | 67.0 | + 1 Lap | 10.0 |
| Thierry Boutsen | Arrows - BMW | 66.0 | + 2 Laps | 21.0 |
| Andrea de Cesaris | Minardi - Motori Moderni | 66.0 | + 2 Laps | 22.0 |
| Christian Danner | Arrows - BMW | 66.0 | + 2 Laps | 20.0 |
| Jonathan Palmer | Zakspeed | 65.0 | Out of Fuel | 18.0 |
| Martin Brundle | Tyrrell - Renault | 65.0 | + 3 Laps | 16.0 |
| Stefan Johansson | Ferrari | 64.0 | Turbo | 14.0 |
| Riccardo Patrese | Brabham - BMW | 64.0 | Spun Off | 5.0 |
| Alessandro Nannini | Minardi - Motori Moderni | 64.0 | + 4 Laps | 24.0 |
| René Arnoux | Ligier - Renault | 63.0 | Engine | 13.0 |
| Allen Berg | Osella - Alfa Romeo | 61.0 | + 7 Laps | 26.0 |
| Johnny Dumfries | Lotus - Renault | 53.0 | Electrical | 17.0 |
| Derek Warwick | Brabham - BMW | 37.0 | Engine | 7.0 |
| Alan Jones | Lola - Ford | 35.0 | Tyre | 15.0 |
| Keke Rosberg | McLaren - TAG | 32.0 | Puncture | 11.0 |
| Michele Alboreto | Ferrari | 10.0 | Turbo | 12.0 |
| Piercarlo Ghinzani | Osella - Alfa Romeo | 8.0 | Turbo | 25.0 |
| Philippe Streiff | Tyrrell - Renault | 8.0 | Turbo | 19.0 |
| Teo Fabi | Benetton - BMW | 4.0 | Engine | 9.0 |
| Patrick Tambay | Lola - Ford | 0.0 | Accident | 8.0 |
Question:
who is the constructor when the laps is less than 68, the grid is more than 20 and the driver is thierry boutsen?
Answer:
Arrows - BMW