One Important Feature of the Transport Proteins Used to Package

One important feature of the transport proteins used to package neurotransmitters into vesicles is that

A) they often have low specificity and may transport more than one type of neurotransmitter.
B) one type of protein transports the low-molecular-weight transmitters, and a second type transports neuropeptides.
C) they are usually saturated and serve as the rate-limiting factor for the availability of vesicles.
D) they may also be present in the cell membrane and transport neurotransmitters directly into the extracellular space.
E) they are highly efficient so that little neurotransmitter can be found outside of vesicles.