What is the spacing between GP-zones of Al-Cu alloy?

• A. 25-50 ‚Äö√†√∂‚àö√±
• B. 50-75 ‚Äö√†√∂‚àö√±
• C. 75-100 ‚Äö√†√∂‚àö√±
• D. 125-150 ‚Äö√†√∂‚àö√±