Blank are necessary to explain research findings. variables
theories
symbols
research models

The answer to this question would be theories.

Research finding must be plausible. That means that there is a theory that can explain why did you get that result. Without it, the research findings will be suspicious and it is hard to be falsifiable. Sometimes the findings are just purely coincidental.

Answer d would be the best answer, for he wants to have part in biotechnology. an ekg technician is quite similar.

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