Which of the following is TRUE about the concepts related to business$-$level strategies?

a$.$ "Non$-$price attributes" are emphasized by the firms following a focus strategy.

b$.$ "Cost parity" means that a cost leadership strategy shouldn't be an excuse for firms to provide customers with a bad quality product compared to peers.

c$.$ "Competitive parity" means that a differentiation strategy shouldn't be an excuse for firms to spend too much extra costs to become differentiated. Their extra costs incurred in being unique should not exceed the price premiums customers are willing to pay.

d$.$ "Experience curve" means that firms are likely to learn how to lower the costs and easily achieve a cost leadership strategy as they gain experience with production processes over time.