Given the task definition and input, reply with output. The task is about reading the given context, group and MCQs and then generating answers from the given input. MCQs are multiple choice questions in which a question is asked and few options are given, among which one or in some cases more than one can be the answer.

context: A person is sliding four bricks across four hard surfaces. The first surface is covered with frost, the second surface is covered with concrete, the third surface is covered with grease, and the fourth surface is covered with oil.<sep>group: sliding <sep>question: Which brick slides the shortest distance?<sep>A: frost<sep>B: concrete<sep>C: grease<sep>D: oil

The surface covered with concrete is the hardest for the brick to slide across.