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:
| Order Year | Manufacturer | Model | Fleet Series (Quantity) | Powertrain (Engine/Transmission) | Fuel Propulsion |
| --- | --- | --- | --- | --- | --- |
| 1992-93 | Gillig | Phantom (High Floor) | 444-464 (21) | DD S50EGR Allison WB-400R | Diesel |
| 1996 | Gillig | Phantom (High Floor) | 465-467 (3) | DD S50 Allison WB-400R | Diesel |
| 1998 | Gillig | Phantom (High Floor) | 468-473 (6) | DD S50 Allison WB-400R | Diesel |
| 2000 | Gillig | Advantage (Low Floor) | 474-481 (8) | Cummins ISC Allison WB-400R | Diesel |
| 2002 | Gillig | Advantage (Low Floor) | 482-492 (11) | Cummins ISL Allison WB-400R | Diesel |
| 2010 | NFI | GE40LFR | 300-309 (10) | Ford Triton V10 ISE-Thundervolt TB40-HG | Hybrid |
| 2011 | NFI | C40LFR | 310-329 (20) | Cummins Westport ISL-G Allison WB-400R | CNG |
Question:
what is the fuel propulsion where the fleet series (quantity) is 310-329 (20)?
Answer:
CNG