I approached the problem from a "system of equations" perspective, and I agree with your analysis. My solution is that there are "infinite solutions" but that one of them is not A.
But then I realized that the problem does not specify that the blanket is rectangular, so we have to assume this in order to use our approach.
I am also frankly shocked that so many wise freepers just assume that the blanket sides must be in neat and tidy integer unit lengths, even if one concedes that the blanket must be rectangular. Nothing in the problem precludes a rectangle with a length 4.5 units, for example.
However, I believe we can safely say that the student is "supposed" to imagine a 3x4 rectangular blanket, provided the student is in a public grade school.
This thread is a candidate for the Hall of Fame. It embodies so many qualities that make Freeping a fun and unique experience.
Although the problem doesn’t specify that the blanket is rectangular, I think it’s safe to assume the vast majority of baby blankets are rectangular. I’m going by the fact that I had four little idiots and 100% of their baby blankets were rectangles. Until they ate the corners off.