Are 'short term' and 'long term' fixed expressions?

[Spring]

Test No. incompl/advan-76 "Internet Business", question 10

You must think big in this business and not just think short ......... but what you hope to do in ten years' time.

(a) term
(b) period
(c) length
(d) time

Test No. incompl/advan-76 "Internet Business", answer 10

You must think big in this business and not just think short term but what you hope to do in ten years' time.

Correct answer: (a) term

Your answer was: incorrect
You must think big in this business and not just think short period but what you hope to do in ten years' time.
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Could you explain the difference of the usage of term and period?

[Torsten]

Short term and long term are strong collocations. That means you cannot replace term with period in this word combination.
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Test Of English for International Communication
TOEIC Preparation & TOEIC Vocabulary