The “mortal blow” (line 51) to Hideki Yukawa’s theory was the fact that 

A) The existence of pions was not confirmed by experimental evidence. 

B) Pions were discovered to be massless, thereby refuting his theory that they were heavy. 

C) Experiments showed pions to be ineffective in the range required by atomic theory. 

D) Pions had a destabilizing effect on atomic nuclei, rather than a stabilizing one.