8. Which item best belongs in place of x? A. they expel hydrogen and sulfide gas B. sea caves are mostly formed in soft rock like sandstone C. crust hardens along the sides first D. fast-moving water carves canyons in caves

## Step 1
This problem is a multiple-choice question that tests your understanding of the process of cave formation. The options provided are all related to the formation of caves, but we need to identify the one that best fits the context of the question.

## Step 2
Option A talks about the expulsion of gases, which is not directly related to the formation of caves.

## Step 3
Option B discusses the formation of sea caves in soft rock like sandstone. While this is a type of cave formation, it is not the most common or significant process.

## Step 4
Option C describes the hardening of the crust along the sides of a cave. This is a crucial step in the formation of caves, as it helps to create the structure of the cave.

## Step 5
Option D talks about fast-moving water carving canyons in caves. While this can happen in some caves, it is not the primary method of cave formation.

## Step 6
Based on the above analysis, option C is the most accurate and relevant to the formation of caves.