Industrialization and modern methods Usage of while and but

[QIAND24]

hi, Ron,
I have a problem about the option C. I noticed, as someone said in the post before, original post has something wrong with option c. After I checked, the option C should be:
"C. while they have introduced 100,000 dangerous chemical pollutants at the same time, which have "

then, i found two mistakes in this option. Please correct me if I made something wrong.
1. "which" -- I am not sure whether it is wrong, but it seems that "which" here modifies "time" incorrectly;
2. the placement of "at the same time" changes the meaning -- the option C means "100000 dangerous chemical pollutants were introduced" "at the same time", however, the original meaning is that the action of "have improved" and the action of "have introduced" happened "at the same time"

Please correct me if I made something wrong. Thanks!

[RonPurewal]

"which" can modify...
...a NOUN,
or
...a NOUN + preposition + other noun.

this version doesn't do either of these things. (it's NOT "noun + prep + noun", because "at the same time" is NOT a modifier of "pollutants".)

therefore the "which..." modifier is wrong.

[RonPurewal]

2. the placement of "at the same time" changes the meaning -- the option C means "100000 dangerous chemical pollutants were introduced" "at the same time", however, the original meaning is that the action of "have improved" and the action of "have introduced" happened "at the same time"

this isn't really a change of meaning, since the same meaning is implied.

i.e., that modifier is stuck on "...pollutants were introduced", but, we need to think: At the same time as WHAT?
clearly the implication is still "at the same time as the improvements". so, nah, that modifier is fine.

the "which" is a glaringly blatant error, so, there's no reason to consider such niceties anyway.

[QIAND24]

Really appreciate your reply!
Actually i have another problem: the correct answer uses "while introducing...". I am confused about this usage, "while + VERBing". Could you clarify that for me? Under what circumstances can we use this kind of structure?
I know that the tenses of two verbs in the structure "SUBJECT + VERB1 + WHILE VERB2-ing" should be parallel, and also the logic subject of VERB2 should be the same as that of VERB1. Anything else to note when we use this structure?
Thanks in advance!

[RonPurewal]

hm?
"__ing" constructions aren't verbs. they don't have tenses.