Which of the following is a sustained competitive advantage for Lululemon?

a$.$ Exercisers' preference for technical athletic fabrics in their sportswear

b$.$ Average return on invested capital during $2008-2017$ of about $31\%,$ far outpacing that of other well$-$known specialty retailers

c$.$ Increasing competitive pressures from Gap's Athleta

d$.$ Sales per square foot estimated to be around $$1,560$