The part of a voltaic cell allows ions to move between half cells so that charge doesn't build up is the ? A. anode.
B. cathode
C. voltmeter.
D. salt bridge.

I believe the answer is salt bridge! Hope this helps.

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The independent variable is the type of music.the dependent variable is the pulse rate.the intervening variable is the age of the subject.if the mother does not like rock, her pulse is likely to not increase.the same might be true for the sister.the friend may like rap and gets excited.to establish cause/effect relationship, all of the subjects must be consistent.hope i .