question: biogas:methane:gas
choices.label: A B C D
choices.text: hydrochloric acid:sodium chloride:liquid thermometer:mercury:liquid rust:fe2o3:solid water vapor:water:water droplets
explanation: "biogas" includes "methane", and "biogas" and "methane" are both "gases". "hydrochloric acid" is an aqueous solution of hydrogen chloride. "hydrochloric acid" and "sodium chloride" have no obvious logical relationship. "mercury" is part of a "thermometer" which is not "liquid". "rust" includes "fe2o3"."rust" and "fe2o3" both are "solid". "water vapor" is the gaseous state of "water".
relation: biogas methane R2.3
relation: mercury thermometer R2.3
relation: rust fe2o3 R2.3
relation: water vapor water R3.10