android-my transparent activity has issue

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http://stackoverflow.com – I have three activity: A,B and C. and the B activity is transparent.and the change order of activitys is A→B→C,then C→B→A.and at the procedure c→B,there happen a strange phenomenon. if i click the screen happen to be on some buttons of activity A,then the A button will be trigger.isn't that werid,because the B activity is transparent so the function are Invisible? and I find if B activiy is not transparent,then this wouldn't happen.why the click will through the transparent B and act on low layer? someone help me plz.:D (HowTos)