meaning of the collocation 'Under stress'

[Izabella]

English Error, Elementary Level

ESL/EFL Test #9 "Sorry, Wrong Number", question 9

Well, to tell you the truth, I'm below a bit of stress at the moment. Sorry to have bothered you. I'll try again.

(a) to tell
(b) below
(c) have bothered

English Error, Elementary Level

ESL/EFL Test #9 "Sorry, Wrong Number", answer 9

Well, to tell you the truth, I'm under a bit of stress at the moment. Sorry to have bothered you. I'll try again.

Correct entry: under
The error was: (b) below

You have not found the error.
Well, telling you the truth, I'm below a bit of stress at the moment. Sorry to have bothered you. I'll try again.
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why we can't use below instead of under?

thank you
Izabella

[Mister Micawber]

.
'Under stress' is the correct collocation. Here, 'under' means 'subject to the control, effect, etc.', which is not a meaning of 'below'.
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Native English teacher at Mister Micawber's