Read the following sentence.

it is important—actually, it is essential—that the extreme athlete choose the right shoe for the sport.

why are dashes, rather than parentheses or commas, used in this sentence?
a.
dashes are being used because they set off longer, less-related parenthetical asides.
b.
dashes are being used to set off brief, closely-related parenthetical asides.
c.
dashes are being used to set off an aside that deserves emphasis.
d.
dashes are being used to set off an appositive.