This question is from MGMAT CAT. I got this one correct but I have problem with official explanation.

Carbon monoxide levels in the atmosphere grew by enough of an increased percentage during the twentieth century that it began to trap heat radiating from the Earth, and it caused the average surface temperature to rise.
A) Carbon monoxide levels in the atmosphere grew by enough of an increased percentage during the twentieth century that it began to trap heat radiating from the Earth, and it caused the average surface temperature to rise.
B) Carbon monoxide levels in the atmosphere increased by enough of a percentage during the twentieth century that they began to trap heat radiating from the Earth, causing the average surface temperature to rise.
C) Levels of atmospheric carbon monoxide increased sufficiently during the twentieth century to begin trapping heat radiating from the Earth, causing the average surface temperature to rise.
D) Atmospheric carbon monoxide levels increased by a sufficient percentage during the twentieth century to begin trapping heat radiating from the Earth, which caused the average surface temperature to rise.
E) Levels of carbon monoxide in the atmosphere during the twentieth century increased enough to begin trapping heat radiating from the Earth, causing the average surface temperature to rise.

The official explanation says "This choice alters the position of "during the twentieth century", thereby changing the meaning of the sentence. In this choice "during the twentieth century" modifies the carbon monoxide levels instead of describing when those levels "increased." This distorts the meaning by leaving open the possibility that carbon monoxide levels "increased enough" during some other time period (e.g., the 21st century)."

How is that the "Levels of carbon monoxide in the atmosphere during the twentieth century" increased in 21st century. Isn't that in 21st century we will have "Levels of carbon monoxide in the atmosphere during the 21st century"?

What's the OA?

You posted an explanation for a choice but did not say which choice it refers to. Can you clarify?

I'm guessing you meant E, just from my read of the choices.

Here, we have "levels of carbon monoxide in the atmosphere during the 20th century" - you have the word "levels" followed by three prepositional phrases, each of which is referring to the preceding words. Ultimately, it's all talking about the levels. So there were some levels of carbon monoxide in the atmosphere during the 20th century. Let's say the level was 10, just to make it easy to talk about this.

The 20th century encompasses 1900 to 1999. The 21st century encompasses 2000 to 2099. Let's say that the "10" level was measured in 1990, which is during the 20th century.

That level, 10, then increased - but at some time not specified by the sentence. Did the level move up to 11 sometime during the rest of the 1990s? Maybe. But could the level have moved up to 11 in, say, 2001? Sure. And that would be the 21st century, not the 20th.

Essentially, in the other options, "during the 20th century" is modifying the word increase - it's telling us WHEN the increase occurred. In E, it's telling us when we are establishing the baseline level, which then increased at some time AFTER that - but the sentence doesn't say when.

Confusing I know - does that make sense?