Answer:
C) at precisely the same time as the food.
Step-by-step explanation:
In an experiment that is carried out to induce an expected outcome, the two variables are always introduced together. This would help in ensuring that the desired aim is achieved rather than lost.
In the case of the experimenter trying to condition a dog to salivate to light during feeding, The best result would be the introduction of the two variables-light and the dog food together.