Which nuclear structural element's function is pre-rRNA processing and ribosome subunit assembly?

a) Nuclear lamina

b) Nuclear pore complex

c) Chromatin

d) Nucleolus

2. Before nuclear pore complexes were well understood, it wasunclear whether nuclear proteins diffused passively into thenucleus and accumulated there by binding to residents of thenucleus such as chromosomes, or were actively transported andaccumulated regardless of their affinity for nuclearcomponents.
A classic experiment that addressed this problem used several formsof radioactive nucleoplasmin, which is a large pentameric proteininvolved in chromatin assembly. In this experiment, either theintact protein or the nucleoplasmin heads, tails, or heads with asingle tail were injected into the cytoplasm or into the nucleus offrog oocytes. All forms of nucleoplasmin, except heads, accumulatedin the nucleus when injected in the cytoplasm, and all forms wereretained in the nucleus when injected there.
A. What portion of the nucleoplasmin molecule is responsible forlocalization inthe nucleus?
B. How do these experiments distinguish between active transport,in which a nuclear localization signal triggers transport by thenuclear pore complex, and passive diffusion, in which a bindingsite for a nuclear component allows accumulation in thenucleus?