Teacher: In this task, you need to answer basic science questions. For each question, several terms are intentionally masked with ***. This masking can cover very few or many of the question words. Clearly, if masked terms are important, the modified question will be impossible to answer. Please indicate the correct answer with one of the following responses: "A", "B", "C", "D" or "E". While "A"-"D" correspond to the answer options provided in the input, "E" represents "I don't know" for questions that do not provide enough information. Respond via "E" if the question is not answerable. Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D', 'E'.
Teacher: Now, understand the problem? If you are still confused, see the following example:
Question: *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***.  (A) dissolving (B) deposition (C) weathering (D) erosion.
Solution: E.
Reason: All the words are dropped, so the correct answer should be 'E' that stands for "I don't know".

Now, solve this instance: Question: When a *** *** is plucked, sound is *** by  (A) the size of the guitar. (B) the metal on the guitar. (C) the wood on the guitar. (D) the vibrations of the string.
Student:
D.