Sentence Correction

Professor Karkow’s theory fails to explain
the extent to which savings from personal
income has shifted to short-term bonds,
capital-market funds, and other mid-term
investments by the instability in the commodities
market.
A. to which savings from personal income
has shifted
B. of savings from personal income that has
been shifted
C. of savings from personal income shifting
D. to which savings from personal income
have shifted
E. to which savings from personal income
has been shifted

D?

savings is plural so 'has' is out. 'shifting' in C is wrong.

Moreover IMO idiom is "extent to which"

It has to be D for the reasons stated by mohit and neeraj

does "shift" modify the savings or the "extend". I would assume that the extend of savings has shifted is correct. Please let me know the OA.

problem with D is it is not using past perfect tense "have been" is ideally suited for this sentence , Since "savings getting transferred' is a event happening before the publication of prof theory . Let me know if iam missing something