global telegraph business peaked, the number of telegrams

[LyndseyG466]

The following question from CAT:

Since 1929, when the global telegraph business peaked, the number of telegrams delivered annually have decreased from 200 million to only 21,000 last year.


A have decreased from 200 million

B have been reduced from 200 million

C has decreased from 200 million

D has been reduced from 200 million down

E has decreased from 200 million down

The correct answer is C, but I was stuck between C & D.

I orginally was thinking C, but I thought that it may not be completely correct.
When is it appropriate to use passive voice in answer D. The explanation for D states:
The passive voice "been reduced" implies that some external force acted to reduce the number of telegrams delivered. The more concise active voice "decreased" is preferable, since it indicates simply that the change occurred. The phrase "down to" is redundant of "reduced."

I realize that other parts of the answers make C correct, but when speaking of "external force"--- wouldn't the external force be the people sending the telegrams?

My questions:
a. When should I typically use passive voice?
b. Are there specific times when to use decreased vs. reduced?

[StaceyKoprince]

Hi! Sorry for the wait.

On this problem, I wouldn't actually decide based on passive voice myself - passive voice is a slippery issue so I try to avoid having to decide based on that. :) The bigger problem is the redundancy - "reduced" already means to go down, so "reduced...down to" is redundant.

There is a slight meaning "weirdness" to using passive voice in this case, because "the number of telegrams has been reduced..." sounds like someone purposely said, "Hey, we really shouldn't be sending so many telegrams. I'm making an executive decision that we should reduce the number of these." This is in contrast to the idea that it just naturally happened over time, which is probably more likely.

But again, I wouldn't decide based on that (and the real test wouldn't make you decide solely on something like that). I'd go for the more rock-solid redundancy.

When the test makes you choose between passive and active - and there's a rock-solid reason for using one or the other - it'll be something based in actual logic, such as:
(A) The memo was published in the company newsletter.
(B) The memo published in the company newsletter.

Obviously, a memo can't publish itself, so you'd have to use passive here, vs. something like The board published the memo. (Also obviously the real test won't make it quite this basic. :)