More than 2.6 million cubic yards of PCB-contaminated sediment

The sentence below has 2 independent clauses & each of them requires their own subject-verb arrangements

(1) More than 2.6 million cubic yards of PCB-contaminated sediment have now been dredged from the bed of the Hudson River, (2) and the chances of people being sickened by the chemicals decrease daily as the amount of both tainted soil and industrial polluters continue to drop.

From the options below we need to do 2 key things - assign the main verb and clear the noun ambiguity

A) as the amount of both tainted soil and industrial polluters continue to drop (verb present but the noun is uncountable)

B) as the drop continues in both the amount of industrial polluters and tainted soil (No main verb)

C) as the amount of tainted soil and the number of industrial polluters continue to drop Correct (verb present & noun is countable)

D) with the continually decreasing amount of tainted soil and the number of industrial polluters (No main verb)

E) with the amount of tainted soil continuing to decrease along with the number of industrial polluters (Idiom mismatch - with A & B)

My answer is (C). It took me 01:29.

"the chances of people being sickened by the chemicals" may not pass the GMAT SC bar because "the chances of people" is not clear.

(A) while we will have a clearer picture on how to fix (A) later, this option can be eliminated because it seems to indicate that the amount of (both tainted soil and industrial polluters) is being discussed. But the amount is singular and thus does not agree with "continue".

(B) Again, we will have a better idea on how to write a perspicuous version. This option can currently be eliminated for the lack of parallelism between "the amount of industrial polluters" and "tainted soil".

(C) We see the appropriate fix. (AB) contains other issues.

(DE) Be careful with "with" as its meaning can almost always be misconstrued. A perspicacious reader may not have difficulty discerning the intended meaning, but that is no justification for shoddy writing. In (DE), "with" can mean "together with".
Also, "with ... -ing" structure overloads too much meaning into a preposition phrase. Better use a clause where proper subject and verb belong.

A) as the amount of both tainted soil and industrial polluters continue to drop
"the amount of tainted soil" and "the amount of industrial polluters". If we open the parallelism you can see that " the amount of industrial polluters" is wrong because "polluters" is countable

B) as the drop continues in both the amount of industrial polluters and tainted soil
From a parallelism perspective; you would see "in [the amount of polluters]" and "in [tainted soil]". It would be better to have "tainted soil" followed by "the amount". Also the "number of polluters" suits better because "polluters" are countable

C) as the amount of tainted soil and the number of industrial polluters continue to drop
Correct

D) with the continually decreasing amount of tainted soil and the number of industrial polluters
Though usage of "with" is not completely wrong here it may trick a reader to think that "chance" decrease together with "the amount of soil...".
Also notice that it would be more concise and less ambiguous to compare two actions; "chances" decrease because "amount of tainted soil" decrease. However, here, the option conveys the following meaning; "chances" decrease because of "some amount of tainted soil".

E) with the amount of tainted soil continuing to decrease along with the number of industrial polluters
Similar issue in D