The greatest foreign exchange earner for Nigeria before the advent of petroleum was

The greatest foreign exchange earner for Nigeria before the advent of petroleum was agriculture. Agriculture was a major source of revenue for Nigeria, and the country was a leading exporter of crops such as cocoa, rubber, palm oil, groundnut, and cotton. These cash crops were exported to countries like Britain, the United States, and other European countries. Agriculture was the mainstay of the Nigerian economy and played a significant role in the country's economic development until the discovery of oil in the late 1950s.