True or False: Most existing underground transmission lines use welded steel pipe for conduit.

The statement is true because most existing underground transmission lines are indeed constructed using welded steel pipe as conduit. This material choice is primarily due to its durability and strength, which are critical for protecting the electrical components from soil pressures, water, and other environmental factors. The welded steel pipe provides a robust, long-lasting solution that can withstand the rigors of being buried underground while allowing for the safe and reliable transmission of electricity. This makes it a common standard in the industry for underground transmission installations. The other choices would imply a misunderstanding of the prevalent practices in underground transmission infrastructure.