Meaning of 'a mixed bag'

[Joe]

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 holder.
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hi there,

when do we use an expression like this? Thanks.

[Alan]

We say this for any collection of people or things that are of different abilities/qualities.
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English as a Second Language
You can read my ESL story Relative Pronoun