Final answer:
Natural resources depleted over time include natural gas wells and timber. These resources are either fossil fuels or biotic materials that are consumed faster than they can be naturally replaced if not managed sustainably.
Step-by-step explanation:
Natural resources that are depleted over time include resources that cannot be replenished at the rate they are consumed. Two specific types of these resources are fossil fuels and timber. Fossil fuels, such as natural gas and petroleum, are formed from plant and animal remains over millions of years and are consumed at a much higher rate than they can be naturally replaced. Timber represents the trees in forests that, if not managed sustainably, can lead to deforestation and degradation of the ecosystem.
Natural gas wells and timber are examples of natural resources that can be depleted over time. Natural gas is a fossil fuel that is used faster than it is formed. Similarly, timber, which refers to trees, can be depleted if forests are not managed sustainably. Inventory and cash, on the other hand, are not natural resources; they represent economic and financial assets.
Therefore, the natural resources from the given options that are depleted over time are a. Natural gas wells and d. Timber.