Mr. Wu___this math problem____him many times, but hestill doesn’t understand it.

Mr. Wang has fallen ill, .so we must_______________a new teacher to the mountain school to take his place.[ ]

下列四个选项中对加点实词的解释全都正确的一项是（ ）

I’m now in New York with my friend Jenny. We ______ by plane on Monday.

Children who are not active or diet is rich in fat will gain weight quickly.