The graph below shows extinction rates during different periods of Earth's history.How many mass extinction events do scientists commonly infer from these data? 
Definitions:
Marginal Costs
The additional cost incurred by producing one additional unit of a product or service.
Variable Costs
Costs that vary directly with the level of production, such as materials and labor, in contrast to fixed costs which remain constant regardless of production level.
Homogeneous Products
Goods that are identical in quality and features, making them interchangeable in the eyes of consumers.
Indistinguishable
Incapable of being identified as different or distinct, often used in contexts where two or more items appear exactly alike.
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