Over the last century, U.S. real GDP per person grew at a rate of about...

a. 2 percent per year, so that it is now 2 times as high as it was a century ago.

b. 4 percent per year, so that it is now 8 times as high as it was a century ago.

c. 2 percent per year, so that it is now 8 times as high as it was a century ago.

d. 4 percent per year, so that it is now 2 times as high as it was a century ago.