有n个整数存放在数组a中，整数k1与k2且k1<k2。某程序段如下：

i = 1: L = 0: R = n + 1

Do While i < R

If a(i) < k1 Then

L = L + 1

If L <> i Then tt = a(i): a(i) = a(L): a(L) = tt

i = i + 1

ElseIf a(i) > k2 Then

R = R - 1

tt = a(i): a(i) = a(R): a(R) = tt

Else

i = i + 1

End If

Loop

则执行上述代码后，下列说法正确的是（   ）

A、数组a变成升序排列

B、a(L)<=k1

C、a(R)>=k2

D、a数组中大于等于k1小于等于k2的元素个数为R-L-1