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Answer:
C. define all variables
Explanation:
Equations for word problems will depend on the problem. Not all of the equations will be linear equations. Even if they are, they are not necessarily easily written in any particular form, or easily solved when they are written in that form. The nature of the equations will depend on the problem.
One thing that must be done in any case is define all variables. It is usually a good idea to define the units of the variable(s) as well. Sometimes problems are posed with numbers in incompatible dimensions, or they may ask for an answer in dimensions different from those used in the given information. In order to avoid confusion and to be sure you have the desired answer, you need to make sure the variables and their dimensions are compatible with the solution process.