How are we supposed to know that "the mugs" refers to only some mugs instead of all mugs?
I'm glad they at least keep the English natural for us. The Japanese in the English-for-Jpn speakers course is made into super unnatural sentences just for the sake of justifying them requiring "a" and not "the" or vice versa, and what's more, the nuance of the sentence is completely compromised by doing so. (slash rant)
Back to my answer. In real life, you'd know from context. On Duo, either you don't have any way to know, or they have to warp the English just to give you cues for the exercise. Whether there's any point in doing that is a point up for debate.