# In a group $(G,o)$ the elements $a$ and $b$ are such that $a^3 = e = b^3$ when $e$ is the identity element, then $a^7 ob^{-4}$ is equal to :
( A ) $a^2ob$
( B ) $a^{-1}ob$
( C ) $aob^{-1}$
( D ) $aob$