Question 4
Neither curve is a straight line. How does its slope change with time?
The right answer is the one marked above.
The slope is steep at the start and grows gentler afterwards. That is, the rate of reaction falls away as the reaction goes on.
As the reaction proceeds the reactant left over dwindles — both the concentration of the acid and the surface area of the limestone fall. So the number of collisions in unit time falls with them.