yeah, this is a horrible question.
i looked up the source on the american version of google books, and checked out the answer key -- and, oh my sweet Lord, these people have no idea how strengthening/weakening questions work.
for instance, choice (c) is an obvious strengthener -- we don't actually know that the levels have increased without this additional information.
...but the answer key tries to eliminate that choice on the grounds that "it is not sufficient to show that vita-plus caused any differences". this is quite disturbing -- it shows that the authors of this book literally don't understand how this type of question is supposed to work! "strengthen" answers are practically never sufficient to show things, and they generally aren't strong enough to prove that something is true; astonishingly, it appears that the authors of this book don't know this.
in reality, (c) and (e) are obvious strengtheners.
also, you should run away from this book as fast as you can, since it's clear that the authors don't sufficiently understand what they're supposed to be doing.
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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