You are given a science question (easy-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 force produces energy as heat?  
 (A) pull (B) gravity (C) friction (D) magnetic
C

Two atoms of oxygen combine to form pure oxygen gas. What is the electrical charge of the O_{2} molecules?  
 (A) positive (B) negative (C) neutral (D) variable
C

Scientists measured the patterns of spectral lines given off by distant galaxies and concluded that almost all of these galaxies are moving away from Earth. What did the scientists observe directly?  
 (A) a shift toward longer wavelengths of light (B) a shift toward shorter wavelengths of light (C) the absence of wavelengths in the visible light range (D) the compression of wavelengths in the visible light range

A