a. RRs saved about 3 percent of GNP over what it would have cost to ship goods and people by alternative methods.
b. U.S. GNP was reached about 20 years earlier with RRs than it would have been reached had RRs not been developed.
c. U.S. GNP was reached about 40 years earlier with RRs than it would have been reached had RRs not been developed.
d. Alternative forms of transport could have shipped goods and people for the same cost or less than the RRs did, and therefore, RRs generated no extra economic growth.