Talent Show Finals Puzzle

#186

In the finals of a talent hunt show, the task was to dabble through a swimming pool full of acid. There were 7 contestants but only 3 of 7 finalists participated in this task. When the 3 contestants get to the other end of the pool by wading through the acid without having any injuries, the other participants were amazed.
Can you guess what happened there!!!! Why they all were not harmed ??

  Difficulty Level - 3/5

  For Solution : Click Here     
  For Hint : Click Here