So, one of both urns is better for triangle type A participants because it has
3 out of 4 {{subsession.session.winning_color_triangle}} marbles.
And the other urn is better for square type A participants because it has 3 out of 4
{{subsession.session.winning_color_square}} marbles.
Type A participants always see the colour of the marble that is drawn from the
urn they choose and thus, each type A participant can try to find out which urn has more marbles of
their winning colour during the 4 rounds of a given block in which the urns are not shuffled.