Why does the heart automatically adjust the flow of blood to match activity levels? a. extreme activity damages blood cells and the body needs replacements. b. the heart requires more oxygen when exercising. c. it builds up a higher level of oxygen in the blood. d. organs and muscles require more blood under stress.

Poliovirus is an rna virus of the picornavirus group, which uses its rna as mrna. at its 5' end, the rna genome has a viral protein (vpg) instead of a cap. this is followed by anontranslated leader sequence, and then a single long protein-coding region (~7000 nucleotides), followed by a poly-a tail. observations were made that used radioactive amino acid analogues. short period use of the radioactive amino acids result in labeling of only very long proteins, while longer periods of labeling result in several different short polypeptides. what conclusion is most consistent with the results of the radioactive labeling experiment? a) host cell ribosomes only translate the viral code into short polypeptides. b) the rna is only translated into a single long polypeptide, which is then cleaved into shorter ones. c) the rna is translated into short polypeptides, which are subsequently assembled into large ones. d) the large radioactive polypeptides are coded by the host, whereas the short ones are coded for by the virus.