Generally, Over 90% of All Membrane Phosphatidyl Serine (PS) Is

Generally, over 90% of all membrane phosphatidyl serine (PS) is found on the inner monolayer of the cell membrane. Which of the following explains this phenomenon?

A) when PS is found in the outer monolayer, a floppase enzyme is used to move it to the inner monolayer
B) the movement of phospholipids from one monolayer to another is a very slow reaction despite the fact that it works with the concentration gradient
C) under normal conditions within the cell, the activity of scramblase enzymes is relatively low and thus does not allow for movement between monolayers
D) All of the above are correct
E) B and C above are correct