why "set of" is used in this case?

[Kirill]

English Language Tests, Intermediate level

ESL/EFL Test #275 "The holiday of a lifetime", question 5

She had also decided that she would buy herself some new luggage, a ......... of cases that cost a lot of money.

(a) pack
(b) set
(c) pile
(d) group

English Language Tests, Intermediate level

ESL/EFL Test #275 "The holiday of a lifetime", answer 5

She had also decided that she would buy herself some new luggage, a set of cases that cost a lot of money.

Correct answer: (b) set

Your answer was: incorrect
She had also decided that she would buy herself some new luggage, a group of cases that cost a lot of money.
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why "set of" is used in this case?

Kirill

[Torsten]

We use the word 'set' when we talk about a number of items that belong together.
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Test Of English for International Communication
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