"There are five cats on the stair" is my answer incorrectly marked wrong. The Portuguese sentence is singular - escada - It is, indeed, possible to have five cats on a singular stair. This should be fixed in the program. Either singular or plural are correct with respect to the stair.
I thought "escada", in the singular, was ladder - from the examples given much earlier - and that "escadas" was stairs or staircase, or ladders? If "escada" in the singular is also stairs or staircase, what is the difference in meaning in Portuguese between "escada" and "escadas"? (I can understand "ladder" vs. "ladders", but not when it is appropriate to choose the singular over the plural if you want to say "stairs") Thanks!