What was a key finding by Stephen Parke and Tomasz Taylor in the 1980s regarding MHV amplitudes?

A. All gluons have negative helicity at tree level
B. Gluons always have mixed helicity at tree level
C. Certain classes of amplitude vanish at tree level when fewer than two gluons have negative helicity
D. All gluons have positive helicity at tree level
Answer: 