The motion of lipid molecules in a synthetic bilayer can be studied by various techniques. Which of the following has been observed in these systems?

A) Phospholipids diffuse rapidly within and between the two leaflets of a bilayer.
B) An average lipid molecule can diffuse the length of about 2 micrometers in a fraction of a millisecond.
C) The flip-flops are very rare for phospholipids but cholesterol molecules flip-flop more often.
D) Within a bilayer, lipid molecules rarely rotate about their long axis, but diffuse laterally at very high rates.
E) All of the above.