Which of the following statements regarding the long$-$short hedge portfolio approach is correct?

a$.$ None of above

b$.$ One of the shortcomings of the long$-$short hedge portfolio approach is that this approach ignores some information we have.

c$.$ The long$-$short hedge portfolio approach can only construct the SMB portfolio.

d$.$ The long$-$short hedge portfolio approach assumes non$-$linear relationship between return and factors