When to use" it" in the comparison structure

[JaneC643]

For what I learned form manhattan sc book, we use " that" to refer to the new copy in a comparison structure. For example, "If current trends continue, by the year 2010 carbon emissions in the United States will soar to a level more than one-third higher than that in 1990, according to official projections. " In this case, we use "that" to refer to a level. But why we cannot use" it" to refer to a level? Is it because "it" means the level in 2010, but it is intended to refer to the level in 1990? If this is ture, in another example, "Since 1990 the global economy has grown more than it did during the 10,000 years from the beginning of agriculture to 1950", we use "it" rather than" that". What's the difference between two examples? When we could use" it" and when we have to use "that"?
Thank you!

[RonPurewal]

"It" has to refer to a noun in the original context.

If the sentence contains "...soar to a level...", then this "level" is the level after the increase (since the statistic soared TO this level).
So, we can't use "it" to refer to the level before the increase.

[RonPurewal]

The pronoun that/those, on the other hand, exists expressly for this purpose: to refer to a noun in a context that differs from that of the original.
(Note the use of "that" in the above sentence, to refer to a DIFFERENT context.)

Since comparisons always involve things that differ in some way, you'll see that/those quite often in comparison sentences.

[RonPurewal]

This thread is now locked, by the way, since it concerns a problem from the free GMAT Prep software.

This problem is already the subject of threads in the GMAT Prep folder. If you have further questions about this problem, please search the forum for one of those threads, and post in it.

Thanks.