单项选择题

下列哪个保留字可以定义常值变量,说明这个变量赋值后永不改变

A. a=5:b=8 Do  Print"*"  a=a+l Loop While a<b
B. a=5:b=8 Do  Print"*"  a=a+l Loop Until a<b
C. a=5:b=8  Do Until a-b   Print"*"   b=b+1  Loop
D. a=5:b=8 Do Until a>b  Print"*"a=a+1 Loop