1.答案：D.
2.翻译：饭后跑步是一种坏习惯.
3.
1）这是一种英语中最常见的句型.
It be + 名词/形容词 + (for/of) sb to do sth.
对某人来说做某事是什么样的.
2）句型中,it是形式主语,不定式才是真正的主语.
如：It's time (for you) to have dinner.
It's a waste of money to buy such an expensive car.
It's difficult to finish the work in five minutes.
It's kind of you to help me.