Answer: OPTION D.
Step-by-step explanation:
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The compete exercise is: "James used substitution to solve this system of equations. Which could be the result of his first step</h3><h3>
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Given the sytsem of equations:

1- Solve for one of the variables from any equation and substitute it into the other equation.
2- Solve for the other variable in order to get its value.
3- Substitute this value back into one of the original equations. and find the value of the other variable.
In this case youu can observe that the second equation is already solved for "y", therefore, in order to use the Substitution Method, the first step is: Substitute the second equation into the first one.
Therefore, the equation that could be the result of James' first step is:

The factors that are needed to be controlled are :
1. the date of manufacturing of the drug should be the same
2. the amount of drug should be the same
3. the time they will take the drug should be the same
4. the food they are taking should be the same
5. the activities they are doing should be the same