By combining the amount of plastic entering the ocean in each country

"by combining the amount of plastics entering" in A sounds wrong, shouldn't this be combining the observations?

VERITAS PREP OFFICIAL SOLUTION:

This complex sentence is mainly testing the location of several different types of modifiers. (A) and (D) start with the prepositional phrase “by combining” while (B), (C), and (E) start with noun phrases. Starting with the “By combining” in (A) and (D), you see that “a known quantity from previous researchers’ work” is an appositive modifier giving extra information about the amount of plastic – it is correctly used, so get rid of it using slash-and-burn to look at the rest of the sentence. (A) correctly says “By combining this amount WITH their own observations, researchers were able…” and (D) incorrectly says “By combining this amount AND WITH their own observations, researchers were able…” . (A) appears to be the correct answer but you must first check (B), (C), and (E).

All of these jumble up the order of modifiers to create meaning and/or structural problems: (B), (C) and (E) create illogical predication as the amount of plastic did not allow the researchers to do anything – it was their act of combining the two data figures that allowed them to estimate. (E) also has the incorrect “and combined with”. Therefore, the correct answer is (A).

Can someone please further explain option B? Ruled out D and E because of 'and'. From the above explanation- does it not make sense to say- A known quantity... combined with their own observation, allowed researchers (as in option B)