More than 300 rivers drain into Siberia’s Lake Baikal, which holds 20 percent of the world’s fresh water, more than all the North American Great Lakes combined.

Why this statement is correct?

My first question is what it is modifying?

-Siberia’s Lake Baikal

-World’s fresh water                            

or

-20% of the world’s fresh water

Please explain the answer as well as reason why it didn’t modify other things.

Note: If you think logically, it will modify 20% of the world’s fresh water but it can modify other nouns too. Hence there is an ambiguity. This question is testing on applying correct modifier. I eliminated this choice bcos i thought there is an ambiguity