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.

When the cardboard cone of a radio speaker vibrates, it produces a sound in the air around it. The cone produces this sound by transferring which of the following to the air around it?  
 (A) heat (B) current (C) wave motion (D) magnetic force
C

On a field trip, a student holds a frog and reports that it has slippery skin. This is an example of  
 (A) a measurement (B) a prediction (C) an explanation (D) an observation
D

While hiking last year, Mike saw a large boulder next to a mountain trail. The boulder had no cracks. While hiking on the trail this year, he saw two large cracks in the boulder. Which of the following most likely caused these cracks to form?  
 (A) shaking from high winds (B) pressure from flowing water (C) erosion due to falling rain and snow (D) weathering due to freezing and thawing

D