A with T: the purine adenine (A) always pairs with the pyrimidine thymine (T)
C with G: the pyrimidine cytosine (C) always pairs with the purine guanine (G)
A molecule is a group of bonded atoms. Answer a, b, and c all have molecules. For example H2O has the molecule H2 in it. The only compound that doesn't share this trait is answer d: NaCl.
Condensation goes from a gas to a liquid and precipitation is when it rains