Assertion: Fibrous joints play a significant role in locomotion.
Reason: Fibrous joints have fluid-filled cavity between the articulating surfaces of the two bones.

Options

A. If both assertion and reason are ture and reason is the corect explanation of assertion
B. If both assertion and reason are ture but reson is not the correct explanation of assertion
C. If assertion is true reason is false
D. If both assertion and reason are false

Explanation

To analyze the given assertion and reason, we will break down the concepts of fibrous joints and their role in locomotion.

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Fibrous joints play a significant role in locomotion."
- Fibrous joints are a type of joint where the bones are connected by dense connective tissue. They are generally immovable or allow very little movement. Examples include sutures in the skull. Therefore, fibrous joints do not facilitate significant movement or locomotion.

2. **Understanding the Reason**: The reason given is that "Fibrous joints have a fluid-filled cavity between the articulating surfaces of the two bones."
- This statement is incorrect. Fibrous joints do not have a fluid-filled cavity. Instead, they are characterized by the absence of a synovial cavity. Fluid-filled cavities are found in synovial joints, not fibrous joints.

3. **Conclusion**: Since both the assertion and the reason are incorrect, we can conclude that the assertion is false and the reason is also false.

4. **Final Answer**: Both the assertion and the reason are false. Therefore, the correct option is D: Both assertion and reason are false.