9-to-1 multiplexer

Build a 16-bit wide, 9-to-1 multiplexer. The 4-bit select chooses one of nine data inputs:

• sel = 0 selects a, sel = 1 selects b, … sel = 8 selects i.
• For the unused cases (sel = 9 through 15), drive all 16 output bits to 1.