Shear modulus is zero for

Options

A. solids
B. liquids
C. gases
D. liquids and gases

Explanation

To determine when the shear modulus is zero, we need to understand the concept of shear modulus and its relation to the properties of materials.

### Step-by-Step Solution:

1. **Understanding Shear Modulus**:
- Shear modulus (also known as rigidity modulus) is a measure of a material's ability to resist shear deformation. It is defined as the ratio of shear stress to shear strain.

2. **Defining Stress and Strain**:
- Shear stress is the force applied parallel to the surface of a material, while shear strain is the measure of deformation representing the displacement between particles in a material.

3. **Types of Materials**:
- Materials can be classified into solids, liquids, and gases. Solids have a definite shape, while liquids and gases do not.

4. **Behavior of Solids**:
- In solids, when shear stress is applied, they deform and change shape, which means they have a non-zero shear modulus. Solids can resist shear stress due to their definite shape and structure.

5. **Behavior of Fluids**:
- In contrast, fluids (which include liquids and gases) do not have a definite shape. When shear stress is applied to fluids, they flow and do not maintain a fixed shape. Therefore, they do not exhibit shear deformation in the same way solids do.

6. **Conclusion**:
- Since fluids do not resist shear stress in a way that leads to a measurable change in shape (as they flow), the shear modulus for fluids is considered to be zero. This applies to both liquids and gases.

### Final Answer:
Shear modulus is zero for fluids (liquids and gases).