Why can't we use "slap" here?

English as a Second Language English Teacher Explanations (ESL Tests)
1

Synonyms Lesson, Advanced Level

ESL/EFL Test #109 [color=blue]“Synonyms for beat”, question 6

The fast wind was … the window-pane and that sound was driving her crazy.

(a) hitting
(b) whipping
(c) pounding
(d) slapping

Synonyms Lesson, Advanced Level

ESL/EFL Test #109 [color=blue]“Synonyms for beat”, answer 6

The fast wind was whipping the window-pane and that sound was driving her crazy.

Correct answer: (b) whipping

Your answer was: [color=red]incorrect
The fast wind was slapping the window-pane and that sound was driving her crazy.
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why cant we use “slap” here ?

(like in “to slap a door”)

thanks!

2

This question is a bit weird and I think that some people might choose one of the other choices, but probably not “slapping”.

“Slapping” is usually used for hitting with something flat, especially the palm or back of your hand. However, other things can slap too. The wind caused the open gate to continuously slap against the outside of the house.