What is the purpose of rate decoupling for ATM transmission?

What is the purpose of rate decoupling for ATM transmission?

A.
It provides transparency between SONET and SDH.

B.
It converts higher-speed DS3/E3 to lower-speed optical rates.

C.
It processess the packing and unpacking of frames into 53-octet cells.

D.
It allows lower-speed traffic on an ATM facility without causing traffic gaps.