From 2009 to 2010 Per Capita Real Gross Domestic Product

From 2009 to 2010 per capita real gross domestic product (GDP) in the United States grew by 1.8%.At that rate,according to the Rule of 70,in roughly how many years will per capita real GDP double?

A) 29 years
B) 68 years
C) 72 years
D) 39 years
E) 18 years