ANS:C - thickness & outer diameter both

A tube is specified by its inner diameter. Here's why:

1. Definition of Tube: A tube is a cylindrical structure that typically has a hollow interior. It is characterized primarily by its internal dimensions, specifically the inner diameter (ID) and the length.
2. Specification: When referring to a tube, the key parameter used to describe its size and capacity is the inner diameter (ID). This dimension determines the flow capacity, volume, and compatibility with fittings and other components in a system.
3. Outer Diameter (OD) and Wall Thickness: While the outer diameter (OD) and wall thickness are also important parameters, they are secondary to the inner diameter in terms of defining the tube's size and functionality. The wall thickness and outer diameter together determine the mechanical strength, weight, and durability of the tube.
4. Common Specifications: Tubes are often designated by their nominal inner diameter (ID), which indicates the approximate internal size of the tube. For example, a tube specified as "1/2 inch (ID)" refers to the inner diameter, not the outer diameter or wall thickness.