In this task, you are given a multiple-choice question and you have to pick the correct option. Answer with option indexes (i.e., "A", "B", "C", and "D").

Thick feathers can be used for (A) climbing, cracking nuts and peeling fruit skins. (B) courtship and looking for food (C) killing prey, preening and feeding young (D) staying weatherproof, flexible and free of bacteria
D