51. Opcode Decoder

How do you plan to solve it?

module alu_decoder (
    input  [1:0] opcode,
    output reg [3:0] alu_op
);
    always @* begin
        /*
        case (opcode)
            2'b00 : alu_op <= 4'b0001 ;
            2'b01 : alu_op <= 4'b0010 ;
            2'b10 : alu_op <= 4'b0100 ;
            2'b11 : alu_op <= 4'b1000 ;
            default : alu_op <= 4'b0000;
        endcase
        */
        alu_op = 4'b0000 ;
        alu_op[opcode] = 1'b1 ;
    end
endmodule