13. let f: r r be defined by f{t) = t and let u be any non-empty open subset of r then (a) fu) is open (b) f(u) is open (c) fu) is closed (d)fu) is closed

13. Let f: R R be defined by f{t) = t and let U be any non-empty open subset of R Then (A) fU) is open (B) f(U) is open (C) fU) is closed (D)fU) is closed