The structural RNA of ribosomes is a

A) pre-mRNA.
B) proenzyme.
C) gene.
D) ribosomal RNA (rRNA) .
E) codon.

The core of a virus contains

A) a membranous envelope.
B) both DNA and RNA.
C) either DNA or RNA.
D) a protein capsid.
E) a protein spore coat.

Which of the following statements concerning solute movement in the loop of Henle is true?

A) Only Na⁺ diffuses from the descending limb.
B) Solutes are actively transported from the ascending limb.
C) Na⁺ and Cl⁻ ions are actively transported into the vasa recta.
D) The osmolality of the descending limb decreases as filtrate moves through.
E) Water moves by osmosis out of the ascending loop.

The "pacemaker" of the heart is the

A) right bundle branch.
B) left bundle branch.
C) AV node.
D) SA node.
E) PM node.