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.

According to cell classification, prokaryotic cells are separated from eukaryotic cells. Which feature is often used to distinguish prokaryotic cells from eukaryotic cells?  
 (A) life processes (B) size differences (C) plasma membranes (D) energy molecules
B

Which pair of organisms benefits each other the most?  
 (A) birds building a nest in a bush (B) worms eating the roots of a plant (C) squirrels burying seeds from a tree (D) deer scraping their antlers on a tree
C

Which of the following best describes a change in Earth's atmosphere made by early photosynthetic life?  
 (A) increased level of oxygen (B) increased level of carbon dioxide (C) decreased ability to support life (D) decreased ability to transmit light

A