In this subtask, you will answer a simple science question. Please indicate the correct answer. If you're not sure about the answer, choose the last option "I don't know". Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D', 'E'. The "A"-"D" responses correspond to the answer options mentioned in the input. There is a 5th option "E" which should be used for questions for which you're not sure about the answer (e.g., when the questions do not provide enough information to answer).

[EX Q]: Question: A pencil falls off of a desk. What moves the pencil toward the floor?  (A) mass (B) friction (C) gravity (D) spee?
[EX A]: C.

[EX Q]: Question: Which is the best example of recycling?  (A) using low energy appliances (B) washing and reusing plastic cups (C) using empty milk cartons as flower planters (D) using aluminum cans to make new product?
[EX A]: D.

[EX Q]: Question: Which of the following is likely to cause the most erosion?  (A) a river (B) a small creek (C) a pond (D) a puddl?
[EX A]:
A.