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.

What type of galaxy is the Milky Way?  
 (A) spiral (B) elliptical (C) irregular (D) oval
A

What type of force keeps the planets orbiting the Sun?  
 (A) gravitational (B) magnetic (C) electrical (D) nuclear
A

Scientists often use mathematical models and computer simulations to study systems. What is the main advantage of using these techniques for studying the spread of infectious diseases?  
 (A) to reduce the amount of work for all involved (B) to reduce the cost of conducting experiments on people (C) to obtain results quickly in a short period of time (D) to perform experiments that would be irresponsible using human subjects

D