To be valid, an experiment must not include a. only one variableb. a controlc. both a control and a variable, which are treated in paralleld. more than one variable

To be valid, an experiment must not include more than one variable.

When carrying out an experiment it is very important to have more than one variable, because this will help to arrive at a good conclusion.

When carring out an experiment, there is usually three kinds of variables, these variables are;

Therefore, option D is correct.

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answer; proximity;

answer; vygtosky's theory;