Is centimeter an S.I unit?
Yes.
https://www.google.com/search?source=hp&q=si+unit+centimeter&oq=+S.I+unit+cen&gs_l=psy-ab.1.1.0i22i30k1l4.12660.20648.0.24998.7.5.2.0.0.0.163.771.0j5.5.0....0...1.1.64.psy-ab..0.7.785...0.F_Quw_AMFB0
https://www.google.com/search?source=hp&q=si+unit+centimeter&oq=+S.I+unit+cen&gs_l=psy-ab.1.1.0i22i30k1l4.12660.20648.0.24998.7.5.2.0.0.0.163.771.0j5.5.0....0...1.1.64.psy-ab..0.7.785...0.F_Quw_AMFB0