Is the ‚k’ in „ciężkich” actually silent? When playing it using the “slow button” Duo doesn't pronounce it at all. I tried to look it up elsewhere and found „ciężki” with the ‚k’ pronounced (at https://en.wiktionary.org/wiki/ciężki), but I would easily believe that the genitive case changed how the ‚k’ was pronounced so that reference didn't reassure me.
As an English speaker I totally accept that words can be spelled in ways that don't match their pronunciation (and it seems they're mostly the exception in Polish), but I just don't trust Duo's accent!
As listening exercises are concerned, we currently block them if the 1st and 3rd person forms only differ by ę/e and there is no subject specified in a different way. It's blocked here as well. There's a slight chance I may have blocked it after reading your comment, but I don't remember opening that sentence in Incubator recently.
What can I say... There's nothing more I can do here. Maybe it just needs some time to get applied. I suggest that next time you can just answer without "the", not to get annoyed... and if you decide to do it with "the" and it's still rejected, could you take a screenshot, upload it on some hosting website and link it here?
"lodówek" is genitive feminine plural. The full declension for "lodówki" is: 1. mianownik/nominative (kto? co?) - lodówki 2. dopełniacz/genitive (kogo? czego?) - lodówek 3. celownik/dative (komu? czemu?) - lodówkom 4. biernik/accusative (kogo? co?) - lodówki 5. narzędnik/instrumental ((z) kim? (z) czym?) - lodówkami 6. miejscownik/locative (o kim? o czym?) - lodówkach 7. wołacz/vocative (o!) - lodówki!