Which of the following statements concerning U.S. airframe manufacturers is not correct?

a. In 1980 there were 29 U.S. manufacturers and 15 foreign manufacturers of piston-engine aircraft. By 1992, there were only 9 U.S. firms and 29 foreign manufacturers.

b. The U.S. has been a net importer of aircraft under 33,000 pounds since 1981.

c. The emphasis by U.S. manufacturers in recent years has almost entirely been multi-engine piston aircraft.

d. In 1992 a foreign aircraft manufacturer sold more single-engine airplanes than any U.S. manufacturer.