Lab1 mask.shape != dots.shape

In lab1, why the mask.shape = (1,2,2), 3 dimensional, while the original dots.shape = (2,2), 2 dimensional? Why the masked dots is 3 dimensional like the mask? why wouldn’t they have the same 2-dimensional shape?

In the current version of the notebook, mask.shape is (2,2). so this appears to have been resolved.