Only variable costs can be differential costs. Do you agree? Explain.

No they are not the same things. So I don’t agree at all and explain it bellow.

Differential cost: Differential cost is the difference between the cost of two alternative decisions, or of a change in output levels.  The concept is used to reach decisions about which alternatives to pursue, and which to drop. The concept can be particularly useful in step costing situations, where producing one additional unit of output may require a substantial additional cost.

 Variable cost: A corporate expense that varies with production output. Variable costs are those costs that vary depending on a company's production volume; they rise as production increases and fall as production decreases. Variable costs differ from fixed costs such as rent, advertising, insurance and office supplies, which tend to remain the same regardless of production output. Fixed costs and variable costs comprise total cost.

 Differential costs are ones that differ between different alternatives. Differential costs are used interchangeably with the terms avoidable, incremental, and relevant costs. However, variable costs are simply ones that vary with different activity levels. They do not necessarily differ between alternatives.

So, Variable costs can’t be differential cost. That’s  why  I don’t agree with this statement