Which of the following statements are true? 1. Cost behavior is considered curvilinear whenever a straight line is a reasonable approximation for the relation between cost and activity. 2. As activity decreases within the relevant range, fixed costs remain constant on a per unit basis. 3. In account analysis, an account is classified as either variable or fixed based on an analyst's prior knowledge of how the cost in the account behaves. Only statement III is true. All statements are true. Only statement II is true. Only statement I is true