IMO A

Three of the answer choices can be eliminated due to clear-cut grammatical errors:

C) The introductory adverbial clause "Since the start of the space age" indicates the present perfect. This describes something that began in the past and continues into the present. For this reason, the simple past "became" is incorrect. It could be replaced by the present perfect "have become".

D) The singular gerund "littering" is incorrectly paired with the plural verb "have"

E) This choice is just all-around awkward! Instead of the present perfect "have become", we have the awkward construction "had been littering...", which is very rarely correct on the GMAT. The rest of the answer choice is just as mangled.

That leaves A) and B)

A) This one is very subtle. It's incorrect because of the meaning behind the wording. This sentence states that the "littering" occurred "because of the intentional discarding". The implication is that the discarding CAUSED the littering, when in fact, the discarding is the littering itself!

Answer choice B) correctly uses the present perfect "have become" to match the opening adverbial clause. It also correctly matches the plural "have become" with "orbits". Finally, "often from" conveys the correct meaning.

yeah, Rich!! makes sense!! I was worried whether" often from" is idiomatic or not!!

Absolutely! "Often from" is just fine idiomatically.

For instance: "I get some serious headaches, often from the stress of finding the correct answers to GMAT questions."

dear gmatmachoman

plz give explanation for your choice even if it is wrong doestnt matter we all are human beings.
thnaks RAZ for your valuable explanations

raz1024 wrote:Absolutely! "Often from" is just fine idiomatically.

For instance: "I get some serious headaches, often from the stress of finding the correct answers to GMAT questions."

@raz1024 and gmatmachoman

What is the level of this question 700 or 750?