Petroleum is the mainstay of the Nigerian economy because it is the?

Petroleum is the mainstay of the Nigerian economy because it is the country's main foreign exchange earner. This means that Nigeria earns a significant portion of its foreign currency by exporting petroleum and petroleum-based products to other countries. As a result, the government relies heavily on the revenue generated from the oil sector to fund its budget and other developmental projects. In addition, many industries in Nigeria depend on petroleum for their operations, including transportation, power generation, and manufacturing, among others. Therefore, the correct option is that petroleum is the "main foreign exchange earner".