Correct Response: D. The solid part of a comet, the comet nucleus, is composed of a mixture of water ice, dust, rock, and several frozen gases. The significant proportion of frozen gases and ice are responsible for the tail produced as comets approach the Sun. While there is evidence that complex hydrocarbons, soot, and sodium may exist on some comets in small amounts (A), they are not major constituents of comet nuclei. Hydrogen, likely from water, does occur in the comet envelope, and nickel and iron can be constituents of the rock and dust part of a comet, but they are not primary components of most comet nuclei (B). Frozen methane can be one of the gases in comet nuclei and carbon is also present. However, elemental oxygen is not found in comet nuclei and as a group these substances do not make up most of a comet's nucleus (C).
