其它
The original price of a product was $$80$$ dollars, and it\textquotesingle s on sale for 30\% off, this product isdollars cheaper than before． 
Overseas Competition->Knowledge Point->Calculation Modules
the new price is $$70\textbackslash\%$$ of the original price，$$so$$ the new price is $$80\times 70\textbackslash\%=56$$；  $$80-56=24$$．  so choose $$\text{B}$$． 