Past Perf Cont. a Past Continuous

Witam !
czy zdanie :
They had been walking all afternoon when Bill suddenly fell down.

można zapisać bez zmiany znaczenia rowniez w Past Continuous ,czyli:

They were walking all afternoon when Bill suddenly fell down.

Czy też o wyborze Past Perfect Continuous decyduje "all afternoon".

pls answer me =)
thx in advance

wedlug mnie nie można.
they were walking all afternoon i kropka. ALBO
they were walking when Bill...

nie podoba mi sie rowniez zdanie "they had been walking all afternoon when...", a konkretnie uzycie w nim 'all afternoon'. Nie potrafie umiejscowic tego momentu w czasie. Gdyby tam napisano "had been walking for 10 minutes", byloby OK - 10 minut od momentu wyjscia.
Podobnie "They were tired. They had been walking all afternoon" jest OK.