Bug fix for the engines. 

Chronology :
Now the two first bubbles show the right answer.
Before this commit, they just blocked. Even if, they were counted as right, the displayed images weren't the good ones.
It was a problem with .animate(). It's explained here : http://www.screenfeed.fr/blog/jquery-1-5-remedier-au-bug-de-animate-avec-background-position-0296/.

Associate :
First : using help while answers to vanish are in boxes used to disable these boxes forever.
Second : switching an answer from a box to another used to disable the first one forever.