下列代码运行后输出结果是 Defstr B-E Private Sub Command1_Click() 　　B$="123" 　　C="456" 　　Print B+C$ End Sub

A．Private Sub Form_MouseDown(Button As Integer，Shift As Integer,_X As Single，Y As Single) 　　If Button=2 Then PopupMenu al,2 　End Sub
B．Private Sub Form_MouseDown(Button As Integer，Shift As Integer,_X As Single，Y As Single 　　PopupMenu al,0 　End Sub
C．Private Sub Form_MouseDown(Button As Integer，Shift As Integer,_X As Single，Y As Single 　　PopupMenu al 　End Sub
D．Private Sub Form MouseDown(Button As Integer，Shift As Integer,_X As Single，Y As Single 　　If(Button=vbLetfButton)Or(Button=vbRightButton)Then PopupMenu al 　End Sub

A．数据库包括数据库系统和数据库管理系统
B．数据库系统包括数据库和数据库管理系统
C．数据库管理系统包括数据库和数据库系统
D．三者没有明显的包含关系