# Why would rinsing the flask with acid in this experiment affect its accuracy?

The following question is #21 from this test form (PDF link):

The apparatus in Figure 4 was set up to measure the time taken for $$\pu{20.0 cm3}$$ of sodium thiosulfate solution to react with $$\pu{5.0 cm3}$$ of hydrochloric acid in a $$\pu{100 cm3}$$ conical flask at $$\pu{20 °C}$$. The timer was started when the sodium thiosulfate solution was added to the acid in the flask. The timer was stopped when it was no longer possible to see the cross on the paper.

What is likely to decrease the accuracy of the experiment? [1 mark]

A) Rinsing the flask with acid before each new experiment.

B) Stirring the solution throughout each experiment.

C) Using the same piece of paper for each experiment.

D) Using different measuring cylinders to measure the volumes of acid and sodium thiosulfate

The answer given as correct is (A). Why is rinsing the conical flask with acid likely to decrease the accuracy of the experiment? I would imagine that stirring the solution, option (B), introduces error since the rate of stirring is difficult to control.