Which of the following is true for economic spacing?

• A. cost of trusses should be equal to twice the cost of purlins
• B. cost of trusses should be equal to twice the cost of purlins minus cost of roof coverings
• C. cost of trusses should be equal to the cost of purlins plus cost of roof coverings
• D. cost of trusses should be equal to twice the cost of purlins plus cost of roof coverings