Describe vs. mention?

[artic]

Test No. incompl/inter-16 "Letter Writing", question 1

I think it would be a good idea to ......... in your letter that you have worked in that type of business previously.

(a) describe
(b) mention
(c) show
(d) demonstrate

Test No. incompl/inter-16 "Letter Writing", answer 1

I think it would be a good idea to mention in your letter that you have worked in that type of business previously.

Correct answer: (b) mention

Your answer was: incorrect
I think it would be a good idea to show in your letter that you have worked in that type of business previously.
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Why word "describe" doesn't fit in here?

[Alan]

Mention means say here. Describe would need an object.
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English as a Second Language
You can read my ESL story Phrasal Verbs/bring