user
Instruction
Answer the question based on the provided table.


Table
Table Title: Challenger LTD
Table Content:
| Client | Years of Operation | Area of Operation | Country | Services |
| --- | --- | --- | --- | --- |
| Agoco | 1991-current | Sarir field | Libya | Drilling, workover |
| Lasmo | 1993-94 | Wadi Borjuj | Libya | Drilling, workover |
| Waha | 1994 | El Zahra | Libya | Drilling, workover |
| IPC-Dublin | 1997-97 | Kilwa | Tanzania | Drilling |
| OMV | 1997 | Field 103 | Libya | Drilling, workover |
| Fina | 1997 | El Hamada | Libya | Drilling, workover |
| Marathon | 1998 | Manzala field | Egypt | Drilling |
| Khalda/Repsol | 1998-99 | West Desert | Egypt | Drilling |
| Total | 1999-current | El Mabrouk | Libya | Drilling |
| Agiba-Agip | 1999 | West Desert | Egypt | Drilling |
| IPLL | 1999-current | El Naka field | Libya | Drilling |
| SOC | 2000-current | SOC fields | Libya | Drilling |
| Veba | 2000 | Different fields | Libya | Drilling, workover |
| Perenco Oil Co. | 2000-01 | EchiraX Concession | Gabon | Drilling |
| Zueitina | 2001-current | Field 103 | Libya | Drilling, workover |


Question
what is the number of operations by an united states client?


Answer Format
The final answer should be concise and use the following format:
```json
{
    "answer": [
        "answer1",
        "answer2",
        ...
    ]
}
```
rule
["2"]