$A$ silicon specimen is made into a $p-$type semiconductor by doping,on an average,one indium atom per $5 \times 10^7$ silicon atoms. If the number density of atoms in the silicon specimen is $5 \times 10^{28} \ \text{atoms} \ m^{-3}$,then the number of acceptor atoms in silicon per cubic centimeter will be

• A
  $2.5 \times 10^{30} \ \text{atoms} \ cm^{-3}$
• B
  $2.5 \times 10^{35} \ \text{atoms} \ cm^{-3}$
• C
  $1 \times 10^{13} \ \text{atoms} \ cm^{-3}$
• D
  $1 \times 10^{15} \ \text{atoms} \ cm^{-3}$