What is wrong with "By comparing..."

Hi,

I’m preparing for a class tonight and going over the answer key for the in-class exercises. The following sentence has been marked as incorrect:

I would probably change this sentence to read:

The explanation in the back of the book is just gobbledygook to me. It reads:

[quote]By comparing > In terms of/ When we compare

“By comparing” should relate to the subject of the finite verb “have”. But that subject is “Europe, Asia and Africa”, so that does not make sense.[/quote]

Can anyone help me to put that into plain English? Their explanation about “not making sense” does not make any sense to me.

Thanks

“Europe, Asia, and Africa” are not doing the comparing. They are the result of a comparison.

This construction should convey the idea that an agent accomplished something by doing something else.

Someone has to do the comparing. I rewrote the sentence to include “we”, as it is “we” who would do the comparing:

By comparing the proportion of people who live in cities around the world, we can see that Europe, Asia and Africa have the highest percentages.

Thanks for the quick replies. All sorted now.