In order for gas exchange to take place, alveoli must be closely associated with:
a) capillaries
b) villi
c) veins
d) arteries

Which of the following represents a difference between viruses and viroids? a) viruses infect many types of cells, whereas viroids infect only prokaryotic cells.b) viruses have capsids composed of protein, whereas viroids have no capsids.c) viruses have genomes composed of rna, whereas viroids have genomes composed of dna.d) viruses cannot pass through plasmodesmata, whereas viroids can.