复数z＝a+i1?i(a∈R，i是虚数单位)是纯虚数，则实数a的值为（　　）A．2B．1C．0D．-

∵z＝a+i1?i=(a+i)(1+i)(1?i)(1+i)=a?1+(a+1)i2为纯虚数，∴a-1=0，解得a=1，故选B．