SANAM Pronoun Question

[sepandsalimi]

Hi everybody:
as it says in page 30 of Manhatan GMAT prep in SANAM Pronouns (SOME, ANY, NONE, ALL, MOST) We have to consider the object after OF and match the verb with the subject after OF but as you may see in GMAT 800 the first question in sentence correction chapter is:
As of this morning, none of my friends have been able to solve the puzzle contained in last weeks newspaper.
as we can see none is the SANAM prounoun and the verb should be agreed with object after of but the book explain as none is singular the verb should be singular as :
none of my friends has been able to solve

could anyone give me some hints in this regard?

thaxs

[capnx]

hi, give this thread a read:
https://www.beatthegmat.com/manhattan-an ... 46667.html

according to Stacey, in this special case both singular and plural forms are acceptable but singular form is preferred