It is "possible" to walk from B to C, but there's a ton of construction so it may not be practical. If you have more than a small roller it won't be easy since I'm sure there are missing curb cuts and bumpy "sidewalks" in between. If you take the bus and it drops you at C it is also still possible to walk outside (in a covered walkway) to D, which will be much faster than the air-side shuttle bus. But it is also along a path that's half under construction so if you have more than a small roller it'll be a PITA.