#C_5H_10# specifies #"the one degree of unsaturation..."#, which specifies an olefinic bond or a ring junction. I think there are FIVE cyclic isomers with this formula: #"cyclopentane,"##"methylcyclobutane, ethylcyclopropane,"##"1,1-dimethylcyclopropane, and 1,2-dimethylcyclopropane."#
And #"1,2-dimethylcyclopropane"# can give rise to TWO geometric isomers, which are?