Which of the Following Statements About Matter-Antimatter Engines Is Not

Which of the following statements about matter-antimatter engines is not true?

A) Matter-antimatter reactions represent the most efficient reactions possible in terms of energy release.
B) Matter-antimatter engines would be possible in theory, but to date, we have no evidence that antimatter exists.
C) One of the major challenges to developing matter-antimatter engines is finding a way to produce enough antimatter.
D) One of the major challenges to developing matter-antimatter engines is finding a way to store antimatter after it is produced.
E) Spacecraft powered by matter-antimatter engines could probably reach speeds of 90 percent of the speed of light.