NOT ONLY BUT ALSO?

[wissmayes]

hi everybody..
I'm confused of this question could you please help me up:
A- China and Russia are also calling for a new foreign-exchange reserves system.
B-Not only China but also Russia are calling for a new foreign-exchange reserves system.
WHY B IS GRAMMATICALLY UNACCEPTABLE?

[bwkcaj_ca]

My grammar book says:

"Singular subjects connected by either/or, neither/nor, and not only/but also require a singular verb. That's because the connecting words show that you are choosing only one item."

Russia and China are both singular subjects

[wissmayes]

SO IT IS : NOT ONLY CHINA BUT ALSO RUSSIA IS CALLING FOR A NEW FOREIGN-EXCHANGE RESERVES SYSTEM?
THANK YOU SO MUCH