Do these actions co-occur?

[Rahel]

English Language Tests, Intermediate level

ESL/EFL Test #173 "Simple Present, Simple Past, Simple Future", question 4

When she was younger, my grandmother ......... in a mortuary.

(a) works
(b) will work
(c) worked

English Language Tests, Intermediate level

ESL/EFL Test #173 "Simple Present, Simple Past, Simple Future", answer 4

When she was younger, my grandmother worked in a mortuary.

Correct answer: (c) worked

Your answer was: correct
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we use when to introduce the action that starts first

Rahel

[Mister Micawber]

.
No, the actions co-occur.
.
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Native English teacher at Mister Micawber's