是垂直于牙面的薄层板状结构（）

A.釉丛

B.釉梭

C.釉板

D.釉牙本质界

E.釉质的生长线

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,然后编写如下程序：Dim S1 As String,S2 As StringPrivate Sub Form_Load()Text1.Text=""Text2.Text=""End SubPrivate Sub Textl_KeyDown(KeyCode As Integer,Shift As Integer)S2=S2&Chr(KeyCode) End Sub Private Sub Textl_KeyPress(KeyAscii As Integer)S1=S1&Chr(KeyAscii)End SubPrivate Sub Command1_Click()Textl.Text=S2Text2.Text=S1S1=""S2=""End Sub程序运行后,在Text1中输入"abc",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．abc和abc

C．ABC和abc

D．ABC和ABC