Cellular components each play a part in overall cellular activities. For instance, ____________ of the endomembrane system performs the critical action of ____________, while ____________ modifies the resulting molecule. Then these molecules contribute to specific functions like reactions, structure, communication, storage, transport, etc. that underlie cellular events.

A. rough endoplasmic reticulum ... assembling proteins ... Golgi apparatus
B. ribosomes ... assembling proteins ... rough endoplasmic reticulum
C. Golgi apparatus ... creating ATP ... mitochondrion
D. nucleus ... protecting DNA ... nucleolus