Simply by observing the existence of the Cherenkov radiation from a particle, one can determine its relative velocity with respect to the light speed in medium. Knowing that, one can determine the particle momentum.
It is a ring formed carbon skeleton. The smallest possible cyclic hydrocarbon is cyclopropane (triangular C3H6) together with cyclobutane both being rather unstable (degration to linear propene and 1-butene). Five and six membered rings are quite common.
Model of a methane molecule. Hydrocarbons are molecules made out of hydrogen atoms and carbon atoms. The simplest hydrocarbon molecule is methane, which is one carbon atom with four hydrogen atoms stuck to it.