Percussion drilling is mainly used where an ______ hole is needed or where ______ earth boring tools cannot be used.

Percussion drilling is particularly effective for creating deep holes, especially in situations where traditional earth boring tools may not be suitable due to specific geological conditions or the need to reach deeper strata. This method utilizes a repeated hammering motion to break up the ground, making it well-suited for penetrating hard rock and dense materials where other tools might struggle or be ineffective.

In contrast, when considering the nature of the other options, extremely shallow holes would not typically require the specialized techniques of percussion drilling, which is designed for deeper applications. Similarly, the term "normal" in this context does not align with the specialized nature of percussion drilling, which thrives in challenging conditions. The mention of "average" is too ambiguous and does not accurately describe the specific advantages percussion drilling has in deep drilling scenarios. Finally, the reference to "unstable" earth and "heavy" tools in the last option doesn't capture the essence of how percussion drilling is applied to reach deep and penetrative holes in dense or hard formations.

Overall, the choice that emphasizes the need for deep drilling capabilities alongside the limitation of conventional tools aligns best with the practical applications of percussion drilling techniques.