which will decrease the rate of a reaction?\nchanging the temperature from 90°c to 40°c\nadjusting the…

Brief Explanations:

The rate of a chemical reaction is affected by factors such as temperature, concentration, surface - area, and volume. Lowering the temperature generally decreases the kinetic energy of reactant molecules, reducing the frequency of effective collisions and thus decreasing the reaction rate. Increasing concentration (from 30 ng to 60 ng) usually increases the reaction rate as there are more reactant particles available for collisions. Placing the substance in a smaller container may increase the pressure (if gas - phase reaction), which can increase the reaction rate. Grinding the substance into a powder increases the surface - area, which also increases the reaction rate.

Answer:

changing the temperature from 90°C to 40°C