Because the majority opinion sets forth an explanation that details the rationale used to formulate the court's decision, and as such becomes part of the body of case law, it provides a valuable reference to be used to predict the outcome of future cases. It also reveals which points of law were most prevalent and the manner in which they were analyzed so that the written law is better able to be interpreted.
A) because it means you have the ability or quality to do a specified thing
Arthur Miller wrote The Crucible in response to the McCarthy hearings in the 1950's as a way of illustrating another, real witchhunt.
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