Which one is classified as a condensation polymer?

Which one of the following bases is not present in DNA?

Sodium phenoxide when heated with $$C{O_2}$$ under pressure at $${125^ \circ }C$$ yields a product which on acetylation products $$C.$$



The major product $$C$$ would be

Let $$f_k\left( x \right) = {1 \over k}\left( {{{\sin }^k}x + {{\cos }^k}x} \right)$$ where $$x \in R$$ and $$k \ge \,1.$$
Then $${f_4}\left( x \right) - {f_6}\left( x \right)\,\,$$ equals :