Magnesium carbonate and dilute hydrochloric acid react to produce magnesium chloride, carbon dioxide and water. $\text{MgCO}_3(s) + 2\text{HCl}(aq) \rightarrow \text{MgCl}_2(aq) + \text{CO}_2(g) + \text{H}_2\text{O}(l)$ To determine the rate of reaction, magnesium carbonate and dilute hydrochloric acid are mixed in a conical flask. The mass of the flask and its contents is recorded every twenty seconds. Which graph shows correctly how the mass of the flask and contents changes with time?