Final answer:
The correct anser is curved lines, which represent the change in direction of the pipes in a simplified, two-dimensional format.
Step-by-step explanation:
In piping isometric drawings, elbows are represented as curved lines. These lines visually depict the change in direction of the piping, simulating the real-world appearance of the elbow fitting in a simplified, two-dimensional manner.
Piping isometrics are a stylized representation, and despite being drawn in a two-dimensional space, they provide a clear and accurate depiction of a three-dimensional piping system.
The use of curved lines helps plumbers, engineers, and construction workers to understand the piping layout without the need for additional details or complex symbols that could complicate the drawing.