Falling energy prices may be welcome news for motorists, but they're bad news for oil producing nations in Africa.
Oil prices have fallen sharply since their peaks in 2008
As the money African countries earn through exporting oil has been hit, investors have been selling many national currencies.
Nigeria, which is the continent's largest oil producer, has been especially hard hit. Its currency, the naira, has fallen by more than 20% against the US dollar over the past two months. Oil exports are Nigeria's biggest single earner of foreign currency
This week, Nigeria's central bank has introduced new exchange controls, limiting the ability of investors to sell the naira in favour of foreign currencies. The Central Bank of Nigeria (CBN) has restricted sales of dollars to companies who could prove they had a transaction requiring them.
Bismarck Rewane, an economist in Lagos, explained what impact the naira's fall was having.
Listen Listen to Nigerian economist Bismarck Rewane (36 secs)
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Economic problems
Razia Khan, an Africa specialist at Standard Chartered Bank, told World Business News's Mike Johnson the underlying reasons for the fall in value of the Nigerian currency.
Listen to Razia Khan (1 min 36 secs)
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