A major principle of budget reviews $($budget vs$.$ actual$)$ is variance analysis. The understanding of the nature of costs, including how they are impacted by activity $($sales$,$ for example$),$ allows one to rationally explain what happened, and plan for potential future variances. Please define and briefly describe the nature of costs, and how they are impacted by variances. Include in your response what, if any control, one has on costs. Provide examples, if applicable.