#### Solution By Steps
***Step 1:***
Distillation separates substances based on their boiling points.
***Step 2:***
Sugar and salt have high melting points and do not vaporize easily, making distillation ineffective for separating them.
***Step 3:***
Oil and water have different boiling points, making distillation effective for separating them.
***Step 4:***
Sand and gravel do not vaporize, so distillation is not suitable for separating them.
***Step 5:***
Water and alcohol have different boiling points, making distillation effective for separating them.
#### Final Answer
B. oil and water
#### Key Concept
Distillation
#### Key Concept Explanation
Distillation separates substances based on differences in their boiling points, allowing for the separation of liquids with distinct boiling points like oil and water or water and alcohol.