whiCh oF thE Following DiD not CAusE so muCh trouBlE For thE mAking oF An AtomiC BomB.'
A．ExtrACtion oF urAnium.
B．sustEntAtion oF A rEACtion.
C．ConvErsion From urAnium orE to urAnium mEtAl.
D．sEpArAtion oF u-235 AnD u-238.