Sentence Correction

Hi,
This probably looks straight forward but its kinda boggled me.

As of this morning, none of my friends have been able to solve the puzzle contained in last week's newspaper.
(A) none of my friends have been able to solve
(B) none of my friends was able to solve
(C) not one of my friends has yet been able to solve
(D) none of my friends has been able to solve
(E) nobody among my friends have solved


Whats the right answer ? scroll down for Kaplan answer and then for why I think this could be wrong.
.
.
.
.
.
.
.
.
.
.
According to Kaplan the answer is D and heres why - None is singular (like every or everyone) and have been is plural. Singular subjects need singular verbs, so we can immediately eliminate (A) and (E). As for (B), Has been beats was. Even though they're both singular, there is no valid reason to replace the original verb. Has been is in the perfect tense and, as such, describes an action that began in the past and continues into the present. The beginning of the sentence (As of this morning) indicates that we're dealing with this sort of situation: the friends have been working on the puzzle, still seem to be working on the puzzle, and are not able to solve it. Was describes an action that took place entirely in the past and therefore is inappropriate considering the beginning of the sentence. So much for (B). As for (C), there's nothing wrong with None that would require its substitution with Nor one (and none is an elision of not one anyway). That leaves (D).


But I thought that None could be singular or plural.

ex- Sing- None of this is true.

ex - Plu - None of my friends are going to make it......as in this question


Let me know what you think.

Thanks,