________ is often the case with children, Tom was completely better by the doctor arrived.

[ ] 

A. That

B. What

C. How

D. As

窗体有命令按钮Command1和文本框Text1,对应的事件代码如下：　　Private Sub Command1_Click( ) 　　For i=1 To 4　　x=3　　For j=1 To 3　　For k=1 To 2　　x=x+3　　Next k　　Next j　　Next i　　Text1.Value=Str(x) 　　End Sub　　运行以上事件过程，文本框中的输出是

A．6

B．12

C．18

D．21