Final answer:
In a well-designed experiment, there should be two or more variables.
Step-by-step explanation:
In a well-designed experiment, there should be two or more variables (C). Having only one variable (B) would not allow for a comparison to be made, and having as few variables as possible (A) may not provide enough information for a meaningful analysis. Having two or more variables allows for the examination of relationships and interactions between different factors.