Concept:The horizontal part of a rate curve shows that the reaction has stopped producing gas.
Explanation:The reaction involved is:
Mg+2HCl→MgCl2+H2.
At point X, the volume of hydrogen gas no longer increases.
Since HCl is in excess, magnesium is the limiting reactant.
Therefore, all the magnesium has reacted completely.
No further hydrogen can be produced, so the curve becomes horizontal.
Answer:C. All the Mg has reacted
