A. One billion is not correct.

Currently, there are about one billion cars, buses and trucks on the world's roads — enough vehicles for one out of every seven people on earth, or about 14% of the world population. Brazil is the country where the current level of vehicle penetration, at 14.9%, is right around the global average.

If one assumes that this global ratio holds steady until 2050, when the world population is expected to grow from the current seven billion to reach 9.3 billion people, the world would then have about 1.3 billion cars, buses and trucks. However, this projection is bound to be far too low.

B. Closer to two billion is not correct.

The International Energy Agency expects car ownership to reach 1.7 billion vehicles in a quarter century from now, by 2035. Much of the growth will be in emerging-market economies.

Typically, car ownership becomes more widespread once a nation's per capita income surpasses $4,000 per year. While the current global median income is only around $1,300 per person per year, it already stands at $7,544 in China and $11,273 in Brazil. India, currently at $3,408, is also within reach of the $4,000 mark, while Indonesia, the world’s fourth-largest nation, is already above it, at $4,347. In Africa, that continent’s most populous country, Nigeria, is also gradually closing in on the mark, with an average income of $2,437.

C. Four billion is not correct.

If the Western European level of vehicle ownership of 583 cars per 1,000 people were to become the global standard, the world would have about 4.1 billion cars — four times more than today. By some estimates, the world is indeed expected to reach this level of car ownership by around 2050.

D. 5.5 billion is correct.

If the entire world had the same rate of car ownership as the United States — 809 vehicles per 1,000 people — there would be about 5.5 billion cars. This is simply unimaginable from a traffic standpoint, as well as from an environmental perspective.

If both China and India had the same rate of car ownership as the United States, they would each add about one billion vehicles to the world's roads.

Stephan Richter presents The Globalist Quiz on U.S. public radio's Marketplace Morning Report. On the January 16 show, he quizzed host Jeremy Hobson on the future of the global car market. You can read the transcript or listen to the audio on the Marketplace web site.