Which of the following transport mechanisms is not rate limited by an intrinsic Vmax（Vmax-最大转运速度）?

Which of the following transport mechanisms is not rate limited by an intrinsic Vmax（Vmax-最大转运速度）?
A.Facilitated di?usion via carrier
B.Facilitated di?usion via channel
C.Primary active transport via pump
D.Secondary active transport via exchanger
正确答案：Facilitated di?usion via channel