统计不同单词个数：在Text1中输入一行单词，单词和单词之间用1个空格或标点符号隔开，单击“统计”按钮，在Text2中输出单词的个数，单词区分大小写，句子以符号结尾，程序运行界面如下图所示，部分程序代码如下：

k=0：s = Text1.Text For i = 0 To Len(s)-1

    ①  

  If ch >= "a" And ch <= "z" Or ch >= "A" And ch <= "Z" Then

    word = word + ch

  Else

    For j = 1 To k

      If    ②    Then Exit For

    Next j

    If     ③    Then

      k = k + 1

      a(k) = word

    End If

    word = ""

  End If

Next i

Text2.text=str(k)

则①②③处的代码为（   ）

A、①ch = Mid(s, i, 1) ②a(i) = word            ③ j <= k

B、①ch = Mid(s, i, 1) ②a(j) = word            ③ j <= k

C、①ch = Mid(s, i+1, 1) ②a(i) = word             ③ j > k

D、①ch = Mid(s, i+1, 1) ②a(j) = word             ③ j > k