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.

[Q]: Which example best shows structures that make up the digestive system in the human body?  
 (A) heart, blood, vessels (B) nose, trachea, lungs (C) brain, spinal cord, nerves (D) esophagus, stomach, intestines
[A]: D


[Q]: Which question about tulips could best be answered by scientific research?  
 (A) Are tulips better than other flowers? (B) What genes determine tulip petal color? (C) Why do people like to look at tulips? (D) Which color of tulips is the prettiest?
[A]: B


[Q]: The ocean's surface is a more varied environment than its depths. Which two water-cycle processes cause the most variation in the ocean's surface salt concentration?  
 (A) transpiration and precipitation (B) precipitation and evaporation (C) evaporation and condensation (D) condensation and transpiration
[A]:
B