Answer: 1. Material being transported 2. The temperature of the material being transported 3. How long the pipes will be there (lifespan)
Explanation: 1. - The pipe builder should consider the material being transported to choose the correct material of the pipes to use, so they last long and can hold the material.
2. - Similar to 1, the pipe builder should consider the temperature of the material to choose the correct material of the pipes to use, so the pipes don’t break and function properly.
3. - If the pipe builder uses a material that can hold the material, that’s great! If that material will last a short time, it would not work if the pipes were to be used for a long time.
The things you said before about viscosity, density, and pressure are also correct. The correct answers are mainly talking about factors of the material being used in the pipes.
hope this helped :)