The answer is:

module Q1 (output reg[7:0] y, input [7:0] A, B, input [2:0] Sel);

always @(A, B, Sel) begin

y=0;

case (Sel)

3'b000: y=8'b0;

3'b001: y=A&B;

3'b010: y=A|B;

3'b011: y=A^B;

3'b100: y=A+B;

3'b101: y=A-B;

3'b110: y=~A;

3'b111: y=8'hFF;

endcase

end

endmodule

module Q();

wire [7:0]y;

reg [7:0] A,B;

reg [7:0] Sel;

Q1 MO (y, A, B, Sel);

initial#180 $finish;

initial fork

#5 begin A=8'hAA; B=8'h55; end

#10 begin Sel=3'b000; A=8'hAA; B=8'h55; end

#20 begin Sel=3'b001; A=8'hAA; B=8'hAA; end

#30 begin Sel=3'b001; A=8'h55; B=8'h55; end

#40 begin Sel=3'b010; A=8'h55; B=8'h55; end

#50 begin Sel=3'b010; A=8'hAA; B=8'hAA; end

#60 begin Sel=3'b011; A=8'h55; B=8'h55; end

#70 begin Sel=3'b011; A=8'hAA; B=8'h55; end

#80 begin Sel=3'b100; A=8'h55; B=8'h00; end

#90 begin Sel=3'b100; A=8'hAA; B=8'h55; end

#110 begin Sel=3'b101; A=8'hAA; B=8'h55; end

#120 begin Sel=3'b101; A=8'h55; B=8'hAA; end

#130 begin Sel=3'b110; A=8'hFF; end

#140 begin Sel=3'b110; A=8'd0; end

#150 begin Sel=3'b110; A=8'hFF; end

#160 begin Sel=3'b111; end

join

endmodule

But I can not understand what it is doing after module Q()......

Can anyone explain to me? Thanks