by the way:
in the context of choices (a) and (b), the dash essentially serves the same purpose as a comma. the only difference is that you don't use dashes at random; you use them when there is some special emphasis or irony (or both) in what you're saying.
example:
Joe, who is from Miami, complained about the cold of the Las Vegas winter. --> no irony; this makes perfect sense, since las vegas is much colder than miami in winter. therefore, no reason to use dashes.
Joe -- who is from Duluth -- complained about the cold of the Las Vegas winter. --> extreme irony, since duluth is about 70 degrees colder than las vegas in the winter.
same sort of deal here: there's considerable irony in the fact that the formidable shark has fewer kills under its belt than does the cute lil yellow fuzzy bee.
as a bonus, the dash also adds a degree of clarity, since "the great white shark" is already followed by one comma + appositive modifier; adding another modifier after another comma would muddy the waters a bit too much for my liking (and, apparently, for the test writers' liking as well).
Ron has been teaching various standardized tests for 20 years.
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Pueden hacerle preguntas a Ron en castellano
Potete chiedere domande a Ron in italiano
On peut poser des questions à Ron en français
Voit esittää kysymyksiä Ron:lle myös suomeksi
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Quand on se sent bien dans un vêtement, tout peut arriver. Un bon vêtement, c'est un passeport pour le bonheur.
Yves Saint-Laurent
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