Mux4Way16 a & b both selected by zero doesn't make sense

[welle036]

The only way I could get the Mux4Way16 to work was by inputing bus a with bus b through the first Mux16 and then c with d in the next Mux16 before combining those outputs thru another Mux16. I dont understand how a and b can be combined when they are both selected by zero. In my head and on paper I need to run a against c, and then run b against d to have proper orientation. But that doesnt work thru the simulator. Any insight?

[WBahn]

It all depends on which control signal you send to which Mux16.