Set expression: Mixed bag

[Robert]

Test No. incompl/elem-23 "On TV tonight", question 2

It will be a bit of a mixed ..........

(a) sack
(b) holder
(c) container
(d) bag

Test No. incompl/elem-23 "On TV tonight", answer 2

It will be a bit of a mixed bag.

Correct answer: (d) bag

Your answer was: incorrect
It will be a bit of a mixed sack.
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Why word sack does't fit in here?

Robert

[Alan]

That's the set expression.

Alan
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English as a Second Language
You can read my ESL story New year resolutions

[Alex Anna]

Hi Alan! "a mixed bag" means various things or many kinds of things?? Do I understand right?

[Alan]

Hi,

Yes, that's right. A mixed bag a variety of things. It is often used to describe things that are not of very good quality or that no one thing is exceptional. You could also use this expression with reference to people as when you have advertised a job and the applicants for the job are referred to as 'a bit of a mixed bag' - very ordinary and no-one outstanding.

A
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English as a Foreign Language
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[Amalmusica]

can we use it only "mixed bag" without the "a bit of"?
is it wrong ?
ex: all these goods are like a mixed bag