'as soon as' vs 'when'

[seema]

English Language Tests, Intermediate level

ESL/EFL Test #142 "Time clauses: future tense examples", question 4

I'll email you ......... I arrive.

(a) as soon as
(b) until
(c) as long as
(d) while

English Language Tests, Intermediate level

ESL/EFL Test #142 "Time clauses: future tense examples", answer 4

I'll email you as soon as I arrive.

Correct answer: (a) as soon as

Your answer was: correct
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why can not we we use "when "?

seema

[Mister Micawber]

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You can use 'when', but that word is not one of the options offered for this question.
.
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Native English teacher at Mister Micawber's