A student wants to create a liquid volcano. The student observes bubbles in the soft drink prior to opening it. The student first proceeds to open the soft drink bottle and place it on the floor.
Next the student drops candy tablets into the drink using a rolled piece of paper, so that the tablets are able to fall into the drink continuously. The student notices a very powerful reaction as the drink fizzes and starts to bubble.

In this scenario, what would be the catalyst ?

The candy tablets being added to the drink.
The gasses being released when the drink was first opened.
The piece of paper being placed over the soft drink.
The container that holds the liquid.