Teacher: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.
Teacher: Now, understand the problem? Solve this instance: 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
Student:
D