某无色晶体，易溶于水.在水溶液中通入H2S气体有黑色沉淀生成，沉淀与H2O2作用转变为白色，该物质是（）A、SnCl2B、SnCl4C、PbCl2D、Pb（NO3）2

编写如下事件过程： Private sub sub1 (ByVal x1 As String, y1 As String) Dim xt As String Dim i As Integer i = Len(x1) Do While i＞= 1 xt = xt + Mid(x1, i, 1) i=i-1 Loop y1 = xt End Sub Private Sub Form. Click() Dim s1 As String, s2 As String s1= "teacher" sub1 s1, s2 Print s2 End Sub 程序运行后，单击窗体，则窗体上显示的内容是

A．rehcaet

B．tahreee

C．themee

D．eerthea

在窗体中添加一个名称为Commandl的命令按钮，然后编写如下程序： Public x As Integer Private Sub Commandl_click() x＝10 Call s 1 Call s 2 MsgBOx x End Sub Pfivate Sub s1() x＝x+20 End Sub Private Sub

A．10

B．30

C．40

D．50

有如下程序： Private Sub Command1 Click() Dim a As Single Dim b As Single a=5：b=4 Call S(a，B)End Sub Sub S(x As Single，y As Single) t=x x=t\y y=t Mod y End Sub 在调用运行上述程序后，a和b的值分别为

A．0 0

B．1 1

C．2 2

D．1 2

写出程序运行的结果

Public class Base

Public virtual string Hello() {return “Base”;}

Public class Sub:Base

Public override string Hello() {return “Sub”;}

1. Base b = new Base(); b.Hello;

2. Sub s = new Sub(); s.Hello;

3. Base b = new Sub (); b.Hello;

4. Sub s = new Base(); s.Hello;

在窗体中添加一个名称为Command1的命令按钮，然后编写如下程序： Public x As Integer Private Sub Command1_Click() x=10 Call s1 Call s2 MsgBox x End Sub Private Sub sl() x=x+20 End Sub Private Sub s2() Dim x As Integer x=x+20 End Sub 窗体打开运行后，单击命令按钮，则消息框的输出结果为( )。

A．10

B．30

C．40

D．50

在窗体上画一个命令按钮和两个文本框，其名称分别为Command1、Text1和Text2，在属性窗口中把窗体的KeyPreview属性设置为True，然后编写如下程序： Diln S1 As String，S2 As String Private Sub Form. Load（ ） Text1．Text="" Text2．Text="" Text1．Enabled=False Text2．Enabled=False End Sub Private Sub Form. KeyDown(KeyCode As Integer，Shift As Integer) S2=S2＆Chr(KeyCode) End Sub Pri vate Sub Form. KeyPress(KeyAscii As Integer) S1=S1＆Chr(KeyAscii) End Sub Private Sub Command1 Click（ ） Text1．Text=S1 Text2．Text=S2 S1="" S2="" End Sub 程序运行后，先后按“a”、“b”、“c”键，然后单击命令按钮，在文本框Text1和Text2中显示的内容分别为（ ）。

A.abc和ABC

B.空白

C.ABC和abc

D.出错