_root always refers to the main timeline you are working with. Think of it as the first floor of a building (with no basement). If you want to get to the 4th floor you have to start at the first floor, go up through the 2nd floor, up through the 3rd floor to get to the 4th right?
Okay, now apply the same to your movie clips. If you are using that code on b and you want to get to c and you want to use absolute addressing then yes, you need to say _root.b.c (assuming that what you are calling ‘a’ is actually your main fla/timeline).
I’m sorry I can’t look at your fla - I can tonight when I get home but that’s not for a bunch more hours so if this doesn’t help clear it up then maybe someone else can look at your file.
By the way, did you try that target tool I mentioned? It really does a lot of the work for you! It looks like what you would see through a scope on a rifle, or those things in movies when one fighter plane is tyring to lock their missiles on another plane. It is located in the horizontal row of tools on the top of the actionscript pane (if you are in expert mode).