Final answer:
Cylinders of compressed gas can be safely loaded either in secure boxes, racks attached to a vehicle, or kept upright/braced laying down to prevent mishaps. Therefore, any of the methods mentioned in the options are acceptable.
Step-by-step explanation:
When handling and loading cylinders of compressed gas, it is important to ensure their safety and stability. They can be securely stored in boxes that won't allow them to turn over, such as specialized gas cylinder boxes. Another option is using specific racks attached to the vehicle which are designed to hold the cylinders securely during transportation. Moreover, they should be either held upright or braced laying down flat, to prevent them from rolling or falling which could lead to a serious accident if the cylinder is damaged. So, the correct answer is D. any of the above
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