This means that for the first time doctors may be able to tell if an infected（被感染的）person who feels well is ________ becoming ill, so that they can try to do something to prevent this

It is important to have your eyes examined regularlyto check for any sign of eye disease that may not have any __________

I don’t like the ________trip, Mary .What about you—I’m also ________ with it .

Our maths teacher set so difficult an examination problem ______ none of us worked out.

Mom asked me to _____________ grandpa when she wasout.