Translation:Why is it so dirty here? Because it is raining.
Why is it here so dirty. This answer should also be right. It is not an English language course, but a Russian language course. Happened before with similar answers.
See my comment above. It's a predicate adjective, not a regular adjective. As such, it's not attached to a noun. In a regular adjective form, it would have to agree with the noun, e.g., Грязная собака - a "dirty dog".
Katzner's Russian-English dictionary says that грязно can be used as an adverb meaning "sloppily" or as a Predicate Adjective meaning "Sloppy, muddy, dirty, filty". The feminine noun грязь means "mud".
So, the fact that it's raining makes more sense in light of these definitions - usually rain washes away a lot of dirt, so that things are cleaner, not dirtier.
Is что part of a grammatical phrase? In other words, can you just say "Потому идёт дождь" or does it have to be "Потому что..."?
Who the fck did these records? It should be грЯзно, not грязнО. It's pronounced completely wrong in every sentence in this course and then new learners learn ❤❤❤❤❤❤❤❤. I'm very disappointed with the audio records. Almost every second one has a wrong stress placement.