Đáp án đúng: B
Giải chi tiết:\(\begin{array}{l}\,\,\,\,\,81:\left( {3 \times 9} \right) = 81:9:3\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \,\,\,\,\,\,\,9\,\,\,\,:3\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \,\,\,\,\,\,\,3\end{array}\)
Chọn B