VFR Corridors through Class B

[U of I Tweak]

Does anyone know what class of airspace the corridors through Class B are? The question came up while giving a lecture on airspace and no one could seem to come up with an answer or find it in the FAR/AIM. Anyway, our assumption is that it is Class E, or at least has Class E cloud clearances. Anyone know for sure and where to find the answer?
Thanks