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:
| Gas name | Chemical formula | Lifetime (years) | 20-yr | 100-yr | 500-yr |
| --- | --- | --- | --- | --- | --- |
| Carbon dioxide | CO2 | See above | 1 | 1 | 1 |
| Methane | CH 4 | 12 | 72 | 25 | 7.6 |
| Nitrous oxide | N 2 O | 114 | 289 | 298 | 153 |
| CFC-12 | CCl 2 F 2 | 100 | 11 000 | 10 900 | 5 200 |
| HCFC-22 | CHClF 2 | 12 | 5 160 | 1 810 | 549 |
| Tetrafluoromethane | CF 4 | 50 000 | 5 210 | 7 390 | 11 200 |
| Hexafluoroethane | C 2 F 6 | 10 000 | 8 630 | 12 200 | 18 200 |
| Sulfur hexafluoride | SF 6 | 3 200 | 16 300 | 22 800 | 32 600 |
Question:
What is the lifetime (years) for chemical formula ch 4?
Answer:
12.0