It could be either. Duo's sentence means "Adults (in general) are responsable"; with the, it means that some specific ("the") adults are responsible.
That is a very good question. I have yet to see any really good answer to this other than the rather vague 'context'. Which doesn't really help us here.
You could differentiate by adding "los" before adultos to indicate you're talking about specific adults and not just adults in general.
Spot on Livione97, and watching politicians' antics is the surest way to verify that!
Silly question perhaps - in English, this sentence could be taken to mean either "adults are reliable," or "adults are at fault." Is the same true in Spanish?
Just to add something I hope is useful: when any consonant (and especially vowels) precede the letter r, it is very helpful in for me to make the sound of that letter as abrupt as possible. Also going through the vowel sounds with a difficult to pronounce word often magically improves my pronunciation.
the male speaker drops his voice on the word "responsables" making it difficult to understand.
Exactly the same as above. There is an ambiguity that exists in the Spanish here that context within the conversation will clarify. All languages have little things like this that require a bit of context to know exactly what is meant.
If context didn't make it clear enough, you could always include "this/that/these/those", if needed, instead of using "the".