Step 1: Understanding the Concept:
This question is about the classification of resources based on their availability, accessibility, and the technology to utilize them.
Step 3: Detailed Explanation:
\[\begin{array}{rl} \bullet & \text{(D) Actual resources: These are resources that have been surveyed, their quantity and quality determined, and are currently being used.} \\ \bullet & \text{(B) Potential resources: These are resources that exist in a region and could be used in the future, but are not yet fully utilized, perhaps due to economic inviability. We have the technology but are not using them yet.} \\ \bullet & \text{(C) Stock resources: These are resources that are known to exist but cannot be used because we do not have the appropriate technology to harness them. For example, extracting hydrogen and oxygen from water (H\(_2\)O) on a large scale for energy is currently technologically not advanced enough, so water can be considered a stock resource for this purpose.} \\ \bullet & \text{(A) Reserve resources: These are a subset of stock resources that can be put into use with the help of existing technology but are deliberately kept for future needs.} \\ \end{array}\]
The question specifically mentions a "lack of technology" as the reason for non-exploration, which is the definition of stock resources.
Step 4: Final Answer:
Resources available but not explored due to a lack of technology are referred to as stock resources.
A project involves eight activities with the precedence relationship and duration as shown in the table below. The slack for the activity D is __________ hours (answer in integer).