Magnetism is believed to be caused by the alignment of small, numerous sub-units called
a. domains
b. forces
c. molocules
d. dominions
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Magnetism is believed to be caused by the alignment of small, numerous sub-units called : Domains

Explanation:

Domains : A magnetic domain is the region in which in which magnetic field of the atoms are grouped together and aligned.

In unmagnetized material all the magnetic Domains  point in different direction.In magnetised material (ferromagnets , antiferromagnets) , The Domains point in a particular( fixed Pattern) direction.

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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 .