A chemist measures a reaction rate as $2.5 imes 10^{-3}$ mol L$^{-1}$ $s^{-1}$. Express this rate in molecules $cm^{-3}$ $\mu s^{-1}$ (molecules per cubic centimeter per microsecond). Avogadro's number = $6.022 imes 10^{23}$ molecules/mol.

Which of the following quantities is dimensionless