local cj = 99
if    90 <= cj <=100  then   
        print("成绩优")
    elseif 70 <= cj <= 89 then
        print("成绩良")
    elseif 60 <= cj <= 70 then
        print("成绩及格")
    elseif 0 < cj <60 then
        print("不及格")
    elseif cj>100 then
        print("成绩有误")
    else
        print("你真是个大笨蛋")
end
这里 90 <= cj <=100  then  这样写不行吗？只能写成这样吗  90 <= cj and cj <=100
        print("成绩优")

是的