$4.($a$)$ The four production and inventory strategies are design$-$to$-$order, make$-$to$-$order, assemble$-$to$-$order, and make$-$to$-$stock. For each strategy, identify the process structure$($s$)$ that is most suitable and explain why.

$($b$)$ Describe the differences between efficient supply chains and responsive supply chains. How should the competitive priorities change for different supply chain designs?