赤のup toと同じ用法を選びなさい。
When I entered the classroom, the children suddenly becae very quiet. I wonder what they were up to.
I left it up to my son to decide what to do about the problem he caused. It's time he learned how to behave.
Long time to see, Mrs.Green! What have you been up to lately?
This is terribly disappointing, but Mrs. Copperfield just isn't up to the job, I'm afraid.
You ca tell when Mrs. Lones is up to something, because he always unconciously opens his eyes a little wider.
up toってなんだ？
そう思っていた時期がカシミア氏にもありましたw