The atomic symbol 206
82 Pb represents lead-206 (Pb-206), an isotope that has 82 protons and 124 neutrons. Which atomic symbol could represent an isotope that undergoes radioactive decay to produce Pb-206?
A.  238 
      92 U
B.  222 
     87 Pb
C.  178 
      72 HfD.  192        77 Pb