The atom? gas? either one of those two would work
A hypothesis is a suggested explanation for an observable problem OR a reasoned prediction of a possible causal correlation among multiple problems. A theory is a tested and well substantiated and unifying explanation for a set of verified and proven factors.
Pulmonary artery carry deoxygenated blood while are there arteries carry oxygenated blood.
pulmonary vein carries oxygenated blood while the other veins carry deoxygenated blood
