Cyclopropane, shown, has no stereoisomers. Treat rings as planar and consider configuration only. Which of the ring compounds shown as options exhibit cis/trans (geometrical) isomerism?

In a ring, cis/trans isomerism arises when two ring carbons each carry a substituent that can lie on the same or opposite faces. P, 1,2-dimethylcyclopropane, and S, 1,3-dimethylcyclobutane, both have two methyl-bearing ring carbons and exist as cis and trans isomers (P: 3 stereoisomers in all since trans is chiral; S: 2, both achiral). Q, 1-methylcyclohexene, and R, cyclohexene, have a ring double bond that is necessarily cis in a six-membered ring and no second substituted sp3 ring carbon, so they show no geometrical isomerism.