Prob in AS in controlling MC inside MC..plz help

prob in AS in controlling MC inside MC…plz help
hi every1…

here i hav attached da .fla file …which is a model of a car game .here da obstacles should come randomly and if da obstacle is going to hit da car the obstacle should disappear …
the prob i face here is …i hav used 3 movieclips( al , bm , cr ) inside a movieclip named “obs”,where any 1 MC from da 3 plays randomly. i hav placed da “obs” MC in da stage…everything is ok…as far as the randomness is cosidered …but the obstacle does not disappear if it hits da car …
i dont know to control da MC (al , bm , cr ) which is inside another MC “obs”. can any 1 help me out 2 solve tis prob … i would really be very thkfull to any1 who hepls me … pleaseeeeeeeeeeee

or i would welcome some other idea to do the same…

thank you …in advance …