（单选题） Which of the following statements is true?

A、 Proteins destined for the ER translocate their associated mRNAs into the ER lumen where they are translated.
B、 Proteins destined for the ER are translated by a special pool of ribosomes whose subunits are always associated with the outer ER membrane.
C、Proteins destined for the ER are translated by cytosolic ribosomes and are targeted to the ER when a signal sequence emerges during translation.
D、 Proteins destined for the ER are translated by a pool of cytosolic ribosomes that contain ER-targeting sequences that interact with ER-associated protein translocators.