问题
单项选择题
Which of the following is an example of inertia
A．A boat can stay on water.
B．A plane can fly in the air.
C．You can walk faster on a very smooth surface.
D．You can go forward and backward on a swing.
答案
参考答案：D
解析：[分析] 推断题型。 见第一段中的最后一句，因此选项D为答案。