Final answer:
Straight perm rods are the rods that maintain a consistent diameter along their entire length or curling area. Unlike their counterparts: spiral, convex, or conventional rods, the diameter of a straight rod remains the same because it undergoes no change in shape.
Step-by-step explanation:
The perm rods that are equal in diameter along their entire length or curling area are D- straight rods. These rods are identical in diameter from one end to the other, unlike spiral, convex, or conventional rods. For example, the figure given illustrates the ideas of tension and compression, which might alter the shape of a rod, but straight rods remain consistently the same diameter, since they undergo no change in shape.
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