A sample of drinking water was found to be severely contaminated with chloroform $(CHCl_3$) supposed to be a carcinogen. The level of contamination was 15 ppm (by mass): express this in percent by mass

Question

$\begin{array}{1 1}1.5\times 10^{-4}\\2.5\times 10^{-4}\\4.5\times 10^{-4}\\5.5\times 10^{-4}\end{array} $