Sentence Correction

Two similar constructions testing same concept-Confused
A question from MGMAT -

What concern scientists the most about global warming are the risks that the polar ice caps will melt, the seas will grow too warm to sustain marine life, and that violent weather patterns may result.

Here is the explanation I found -

From there, the key is to identify the subject. Ask yourself, What concern/concerns scientists? The answer is the risks, a plural subject. So, the verb is ARE

However, if I look at this sentence from OG -VR2

The period when the great painted caves at Lascaux and Altamira were occupied by Upper Paleolithic people has been established by carbon-14 dating, but what is much more difficult to determine are the reason for their decoration, the use to which primitive people put the caves, and the meaning of the magnificently depicted animals.

The correct Answer is -
B) has been established by carbon 14 dating but what is much more difficult to determine is


Here is the explanation I found -

The subject of the quoted clause is the short clause "what is much more difficult to determine". And clause is always Singular. Hence verb should be "IS"

My doubt is -

I found the above two questions testing the same concept.
However, one question i.e VR 2 question taking the subject as "WHAT" and the other one taking the subject as RISKS. Can expert please explain why in one question we are taking the forward construction and in other we are looking at the reverse construction. Please explain.

Thanks