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.

Dr. Tanaka discovered a new virus. The virus has a single strand of a nucleic acid, but she does not know if it is DNA or RNA. After running tests, she concludes that it is DNA. Which of the following would lead her to that conclusion?  
 (A) It contains uracil. (B) It contains adenine. (C) It contains thymine. (D) It contains cytosine.
C

Water in the solid phase has a crystal structure which keeps molecules farther apart than water at certain temperatures in the liquid phase. This relationship explains which property?  
 (A) the presence of more impurities in ice than in liquid water (B) the ability of ice to float on liquid water (C) the release of heat when liquid water turns into ice (D) the cooling of liquid water as it flows over ice
B

Over which of the following components of smog do humans have the least control?  
 (A) water vapor (B) sulfur dioxide (C) ammonia (D) nitrogen oxides

A