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.

The respiratory system cannot deliver oxygen to the cells of the body, nor can it remove carbon dioxide, without which two body systems?  
 (A) muscular and nervous (B) digestive and muscular (C) cardiovascular and nervous (D) cardiovascular and muscular
D

Which of these determines whether a fertilized egg develops into a toad, a snake, or a lizard?  
 (A) the sex of the egg (B) the age of the egg (C) the size of the egg (D) the genes of the egg
D

Coal is formed from  
 (A) seas that have evaporated. (B) natural gas subjected to heat. (C) melted and cooled metamorphic rock. (D) plant remains decomposed under pressure.

D