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.

All planets of the solar system rotate, but not all planets have the same rotational velocity. Which characteristic of a planet is determined by the rotational velocity of the planet?  
 (A) the strength of the gravity (B) the shape of the orbit (C) the size of the planet (D) the length of the day
D

Which factors can have the greatest effect on the health of a river system?  
 (A) type of soil and salinity (B) nitrate levels and turbidity (C) human consumption and pH (D) natural disasters and tidal changes
B

Which question will best help a student determine the composition of an unknown substance?  
 (A) Is the sample warm or cold? (B) How valuable is the sample? (C) Has the sample ever been melted? (D) Does the sample conduct electricity?

D