Southwest Airlines (SWA) has enjoyed a sustained competitive advantage, allowing it to outperform its competitors over several decades. Continental and Delta attempted to copy SWA with their offerings of Continental Lite and Song, respectively. Neither Continental nor Delta, however, was able to successfully imitate the resource bundles and firm capabilities that make SWA unique. Which of the following is this case an example of?

o resource immobility
o resource substitution
o resource heterogeneity
o resource imitation