When external forces are applied to common glass made from silicates, energy builds up around minuscule defects in the
material, resulting in fractures. Recently, engineer Erkka Frankberg of Tampere University in Finland used the chemical ______
to make a glassy solid that can withstand higher strain than silicate glass can before fracturing.
compound, aluminum oxide
compound aluminum oxide,
compound, aluminum oxide,
compound aluminum oxide
Choice D is the best answer. The convention being tested is the use of punctuation around noun phrases. No punctuation is
needed because the noun phrase “aluminum oxide” is a restrictive appositive, meaning that it provides essential identifying
information about the noun phrase before it, “the chemical compound,” and thus doesn’t require punctuation around it.
Choice A is incorrect because no punctuation is needed. Choice B is incorrect because no punctuation is needed. Choice C
is incorrect because the noun phrase “aluminum oxide” is a restrictive appositive. Setting the phrase off with punctuation
suggests that it could be removed without affecting the coherence of the sentence, which isn’t the case.