单选题 assume cs:code data segment db "Welcome To Assembly.",0 data ends code segment start. mov ax,data mov ds,ax mov si,0 mov bx,0 call count mov ax,4c00h int 21h count: mov cl,[si] mov ch,0 jcxz ok cmp byte ptr [si],'a' jb s0 cmp byte ptr [si],'z' ja s0 inc bx s0. inc si loop Count ok. ret code ends end start 上面的程序执行时,count返回前,bx中的值为( )