TASK DEFINITION: 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.
PROBLEM: One hot summer, a grassland experienced a drought. Which most likely happened in the ecosystem during this time?  
 (A) The competition for food among primary consumers increased. (B) Nutrients cycled through the system at a faster rate. (C) Less solar radiation was able to enter the system. (D) Secondary consumers became extinct.

SOLUTION: A

PROBLEM: One substance found on Earth is more stable as an elemental gas than as part of a solid compound. As a result, this substance is very abundant in the atmosphere. Which substance exhibits these traits?  
 (A) argon (B) chlorine (C) nitrogen (D) oxygen

SOLUTION: C

PROBLEM: Sonar can be used to measure distances underwater. This is accomplished by sending a sound wave to a "target" while measuring the total time it takes for the wave to reach the target and then return to its source. Which of the following sound wave characteristics must be known when using sonar to measure distances?  
 (A) amplitude (B) wavelength (C) frequency (D) speed

SOLUTION:
D