Absurd undefined variable

[FONT=Arial]Hello everyone.[/FONT]
[FONT=Arial]I have trouble with an undefined variable.[/FONT]

[FONT=Arial]I have three movieclips ([COLOR=blue]a[/COLOR],[COLOR=blue]b[/COLOR],[COLOR=blue]c[/COLOR]):[/FONT]
[FONT=Arial][COLOR=blue]b[/COLOR] scrolling limits are “controlled” by variable [SIZE=3]“i”[/SIZE] setted in [COLOR=blue]a[/COLOR].[/FONT]
[FONT=Arial][COLOR=blue]c[/COLOR] is a movieclip that must appear only for a certain value of [SIZE=3]“i”[/SIZE] established as follow.[/FONT]

[FONT=Arial][COLOR=blue]a[/COLOR] has this script:[/FONT]
[FONT=Arial]onClipEvent (mouseDown) {[/FONT]
[FONT=Arial]if(this.hitTest(_root._xmouse,_root._ymouse,false) ){[/FONT]
[FONT=Arial]if (_parent.i < 50) {[/FONT]
[FONT=Arial]_parent.scrolling.fine -= 96;[/FONT]
[FONT=Arial]_parent.i++;[/FONT]
[FONT=Arial]}[/FONT]
[FONT=Arial]}[/FONT]
[FONT=Arial]}[/FONT]

[FONT=Arial]In the first frame of main timeline is written:[/FONT]
[FONT=Arial]i=1; // to let limits of scrolling movie [/FONT]

[FONT=Arial]And adding:[/FONT]
[FONT=Arial]if (i=1) {[/FONT]
[FONT=Arial]mc._alpha=100; }[/FONT]
[FONT=Arial]else {mc._alpha=0;}[/FONT]

[FONT=Arial]it doesn’t work![/FONT]

[FONT=Arial][COLOR=blue]c[/COLOR] is visible but, after clicking on a to scroll [COLOR=blue]b[/COLOR], [COLOR=blue]c[/COLOR] remains still visible (while, [SIZE=3]“i”,[/SIZE] that lets scrolling, has changed to 2).[/FONT]

[FONT=Arial]How can I resolve, please?[/FONT]

[FONT=Arial]Thank you very much.[/FONT]