Utilize the input text as a context reference, choose the right relationship between 'Stratasys' and '3D printing' from the options.
Options: brand, product/material produced, distribution format, currency

"We think 3D printing is a significant long-term opportunity, but the technology is immature in terms of speed, quality and reliability, which has caused problems for Stratasys and 3D Systems," Milunovich wrote.
product/material produced