In grain cleaning and grading machinery design:
An air-screen cleaner combines pneumatic aspiration (air stream) to separate light impurities based on terminal velocity and vibrating perforated screens to separate material based on geometric size and shape.
Primary design considerations include:

• Particle size and shape (A) to select screen perforation geometry (round vs slotted holes).
• Stickiness and abrasiveness (C) which govern screen blinding, screen wear, and deck material.
• Flow characteristics (D) which determine feeder design, deck slope, and throughput rate.

Official NTA answer key designates (A), (C) and (D) only.

In grain processing and seed cleaning machinery:
Grain separators classified strictly on the basis of length separation are:

• Indented Cylinder Separator (A): Rotating drum lined with precise hemispherical indents that pick up shorter kernels and drop them into an internal trough while longer kernels reject the indents and discharge at the end.
• Disk Separator (C): Vertical rotating disks with undercut pockets that lift short seeds out of a grain mass based on seed length.
  In contrast, the Inclined draper (B) separates seeds based on shape and surface friction/rollability, while the Rotary screen (D) separates seeds based on width and thickness.

In materials handling and belt conveyor engineering:
When granular materials have a high angle of repose (\(\ge 35^\circ\)), the material forms a stable self-supporting heap without excessive spilling over the belt edges.
In specific conveyor design configurations and standard test bank classifications, Flat belt idlers are evaluated where materials with large angle of repose are carried on flat belts without troughing.
Official NTA answer key specifically adopts Option 4 (Flat belt idlers).