b)decrease of concentration of so**粗俗文字**m thiosulfate will lower the chance for effective collision as the number of reactant molecule so**粗俗文字**m thiosulfate decrease.Decrease the temperature will lower the rate of reaction as the average KE of reactant molecules decrease,they move in lower speed and collide less frequently,thus the
chance for effective collision decrease.Also,there will be less portion of molecules have enough activation energy to allow the reaction occur,thus the chance for effective collision decrease,rate of reaction decrease.