(2017·北京卷·单项填空）The little problems ________ we meet in our daily lives may be inspirations for great inventions.

We were all very much attracted by the beautiful scenery ____ we set foot in the village. [ ]

Each of the students, working hard at his or her lessons,___to go to university.—–So do I.

Tom, Jane wanted you to call her.—I’ll ____ in twenty minutes. Thank you.

I think ____necessary for us _____the birds.