Ahhhhhh, ok! I got it! A lot of thanks! You explication help me a lot! The "key" was the word "leading".
I have a serious problem with english! I looked for "trailing" word, but I could not find the correct meaning.
In a few days I am going to take the real exam, and I chose the enlgish language. I hope that I will not have this problem on the real exam!
==> "trailing": end of the string?
==> "leading" : start of the string?
I can not find this meaning with the sense that I like in the dictionary! Please, let me know if I am right about the sense that I give to this words.