If bromine atom is available in the form of,say,two isotopes $_{35}^{79}Br$ $(49.7 \%)$ and $_{35}^{81}Br$ $(50.3 \%),$ calculate the average atomic mass of bromine atom. (in $,u$)

• A
  $0.80$
• B
  $80.006$
• C
  $89.006$
• D
  $88.006$