选出对于有下划线语句解释正确的释义（        ）
Library ieee;
Use ieee.std_logic_1164.all;  定义元件库              
entity qk_11 is
port( a,b,c,d,en:in std_logic;
      s:in std_logic_vector(1 downto 0);    
      op:out std_logic );
end qk_11;
architecture ar_1 of qk_11 is
    signal f:std_logic_vector(2 downto 0);
begin
f<=en&s;     
  process (f)    
  begin
      case f is
           when"100"=>op<=a;
          when"101"=>op<=b;
          when"110"=>op<=c;
          when others=>op<=d;
end case;
end process;
end ar_1;
A、将输入信号d 赋值给 op
B、敏感信号f的变化将启动进程 process
C、输入信号s是两位的输入总线
D、将输入信号en和s连接赋值给f