You are given a science question (hard-level) and four answer options (associated with "A", "B", "C", "D"). Your task is to find the correct answer based on scientific facts, knowledge, and reasoning. Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D'. There is only one correct answer for each question.

Which of these best describes a change that is physical?  
 (A) yarn being knitted into a sweater (B) the engine from a tractor being started (C) the metal on a bicycle forming rust (D) grass capturing the energy from sunlight
A

What is the complementary base to cytosine in DNA?  
 (A) uracil (B) thymine (C) guanine (D) adenine
C

Which describes only a physical change?  
 (A) baking a cake (B) frying an egg (C) burning a piece of toast (D) squeezing the juice from fruit

D