求助——关于回归方程的变量选择

bwmessi

y<-c(7.8,8.4,8.7,9,9.6,10.3,10.6,10.9,11.3,12.3,13.5,14.2,14.9,15.9,18.5,19.5,19.9,20.5)
x1<-c(81.2,82.9,83.2,85.9,88,99,102,105.3,117.7,126.4,131.2,148,153,161,170,174,185,189)
x2<-c(85,92,91.5,92.9,93,96,95,95.6,98.9,101.5,102,105,106,109,112,112.5,113,114)
x3<-c(87,94,95,95.5,96,97,97.5,98,101.2,102.5,104,105.9,109.5,111,110,112,112.3,113)
试问如何用RMSq，Cp，和AIM准则建立子集回归模型。
PS：我知道用step（）可以建立AIM准则下的回归模型，但是请问前面两个怎么弄。多谢啦。