Millions of west africans who migrated to nigeria during the 1970's, when the country's economy expanded, were expelled during the 1980's, when the country's economy declined. this is an example of:

Pull factor becoming a push factor

Explanation:

Nigeria is the most populous black nation on earth and attracts a lot of tourist as well as investors at every point in time. During the 1970's, there was migration of people from other west African countries due to the economic stabilty and increasing economic expansion, thus making Nigeria a place to search for greener pasture within the continent. In the 1980's, there was an economic downturn that hit the country so hard that Nigerians started calling for the exit of fellow african nationals in the country. Most affected country then was Ghana and there was a slogan with tthe phrase 'Ghana-must-go'.

The phrase went on to become the name of the bags with which Ghanians left tthe country with.

N.B: look up Ghana-must-go bags on google.

Cheers.

answer(b) equal; set period;