Why did we not use fog?

[Meriline]

English Language Tests, Intermediate level

ESL/EFL Test "Life on Mars?", question 9

Unfortunately the dust storms tend to ......... the view of the planet.

(a) fog
(b) obscure
(c) confuse
(d) darken

English Language Tests, Intermediate level

ESL/EFL Test "Life on Mars?", answer 9

Unfortunately the dust storms tend to obscure the view of the planet.

Correct answer: (b) obscure

Your answer was: incorrect
Unfortunately the dust storms tend to fog the view of the planet.
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Obscure hear means what.
Why we have not used fog?

[Alan]

Hi Meriline,

Obscure is make something unclear.

Alan
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English as a Second Language
You can read my ESL story A day in the life of a driving instructor

[Saneta]

why can't it be: darken here? thanks