<p>1.The Empire State Building, the Sears Tower, the
Canadian National Tower[each of these structures] [was] the [tallest]
in the world at the time [they were] built.</p>
<p>Apparently here the answer is "D"</p>
<p>2.The cost of [safely disposing] of the toxic chemicals [is approximately]
[five times what] the company paid [to purchase it].</p>
<p>The answer here is also D, could anyone is explain both answer? Thanks</p>
<p>These two all have problems with singular / plural.
[each of these structures] [was] the [tallest] in the world at the time [they were] built.
The subject is singular, so “they” should be “it”.
The cost of [safely disposing] of the toxic chemicals [is approximately]
[five times what] the company paid [to purchase it].
“It” should be “them”, since the subject (toxic chemicals) is plural.</p>
<p>Although I know that option D is wrong because of the Pronoun-Antecedent Error, I seem to have some trouble with the “safely disposing of” part. It seems to me that since “disposing” is a gerund here (correct me if I’m wrong), shouldn’t “safely” be written as “safe”? I mean gerunds act as nouns, so an adjective is needed instead of an adverb.</p>
<p>@Marstatic You’re half right. Here’s a [good</a> explanation](<a href=“syntactic analysis - Should I modify a gerund using an adjective or an adverb? - English Language & Usage Stack Exchange”>syntactic analysis - Should I modify a gerund using an adjective or an adverb? - English Language & Usage Stack Exchange).</p>
<p>Wow! Thank you for the prompt response! 
@WasatchWriter</p>
<p>wait…I didn’t see the detailed answer by “cerebrus” when I first opened the article. His/her explanation says:
"If you modify a gerund “from the outside”, you treat it as a noun, and so you use an adjective:</p>
<p>That’s quick thinking! (= that is a quick act of thinking)</p>
<p>I heard a faint rustling of feathers or clothes.
When emphasising the nominal aspect of a gerund as above, this normally corresponds with expressing the agent of the action with of. When you use a or an, this forces you to emphasise the nominal aspect." </p>
<p>“this normally corresponds with expressing the agent of the action with of”</p>
<p>According to this explanation, the “safely disposing of” is still wrong. It either should be “The cost of safe disposing of the toxic…” or “The cost of safely disposing the toxic…”</p>
<p>Shouldn’t it? :-/ </p>
<p>@WasatchWriter ?</p>
<p>That’s a different “of.” In “rustling of feathers,” “rustling” is a property of “feathers.” In “disposing of the chemicals,” “chemicals” is the object of “disposing.” It’s a very different relationship.</p>
<p>What I suspect you are noticing is the awkwardness of “disposing of.” There’s nothing intrinsically wrong with it. But I think you will find that when a noun based on a verb stem coexists with a gerund form of the same verb, we usually prefer the noun to the gerund. In this case the noun form is “disposal”; 'the safe disposal of toxic chemicals" sounds enormously better than any variation of the sentence that uses “disposing of.”</p>
<p>Both the SAT and practice books are notorious (among linguists, anyway) for creating problems in which no solution seems correct because a second problem has eluded the attention of the test writer.</p>