"a dozen of bananas" or "a dozen bananas"?

[Kwauk]

English Language Tests, Intermediate level

ESL/EFL Test #652 "English Slang Idioms (394)", question 3

"Is Karl an idiot? He keeps saying that we should reject my plan and adopt his. His is the same as mine! It's six of one and half a ......... of the other!" Greg blurted.

(a) score
(b) pack
(c) case
(d) dozen

English Language Tests, Intermediate level

ESL/EFL Test #652 "English Slang Idioms (394)", answer 3

"Is Karl an idiot? He keeps saying that we should reject my plan and adopt his. His is the same as mine! It's six of one and half a dozen of the other!" Greg blurted.

Correct answer: (d) dozen
_________________________

Is it correct to say a dozen of bananas or a dozen bananas?
Or even both are correct, is there a rule for that?
Kwauk

[Mister Micawber]

Only 'a dozen bananas'.
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Native English teacher at Mister Micawber's