"still asleep" vs "fast asleep"

[Suliko]

English Error, Elementary Level

ESL/EFL Test #21 "Thank you letter", question 4

The postman had to knock a few times because I was quickly asleep at that time in the morning.

(a) quickly
(b) asleep
(c) in

English Error, Elementary Level

ESL/EFL Test #21 "Thank you letter", answer 4

The postman had to knock a few times because I was fast asleep at that time in the morning.

Correct entry: fast
The error was: (a) quickly

You have found the error but your entry is incorrect.
The postman had to knock a few times because I was still asleep at that time in the morning.
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still asleep ... fast asleep?

Suliko

[Mister Micawber]

.
It seems to me that 'still' also works in this sentence; the idea, however, is to replace the incorrect vocabulary with a similar but appropriate word.

In this case, 'quickly' and 'fast', which are synonyms in many structures, are not synonyms here. 'Fast' here means ''completely, firmly'.
.
_________________
Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mr. Micawber