The lower pairs are _________ pairs.

Answer: Option A

Definition:
A lower pair is a type of kinematic pair where two links have surface or area contact with each other during relative motion. This type of contact is more stable and allows motion such as sliding, turning, or screw motion. Examples include revolute pairs (hinges), prismatic pairs (sliders), and screw pairs.

Correct Answer: Option A – Self-closed

Explanation:
In mechanical systems, pairs are classified based on how the contact between the elements is maintained.

Self-closed pairs are those in which the elements are kept in contact by the geometry and arrangement of the system itself, without the need for any external force. This is characteristic of lower pairs, where the connection between the links is inherently maintained by their design — such as a shaft rotating in a bearing or a slider moving in a guide.

Because lower pairs have surface contact and are physically constrained by their shapes (e.g., cylindrical or prismatic), they remain connected throughout the motion without needing extra forces. This is what makes them self-closed.

Why other options are incorrect:
Option B: Force-closed – Force-closed pairs require an external force (like a spring or gravity) to keep the elements in contact. This applies to certain higher pairs, not lower pairs.
Option C: Friction closed – This refers to systems where motion is sustained by frictional forces, such as belts or clutches. This is not a defining feature of lower pairs.
Option D: None of these – This is incorrect because Option A correctly defines the nature of lower pairs.

Conclusion:
Lower pairs are self-closed because their elements maintain contact through their geometric configuration without requiring external forces.