He is really an excellent doctor and he is _____ to becoming an expert in this field.
I often watch movies to practice my spoken English. I think it ______.
—Is Tom at home? —No, he ______ to town.
I hate traveling by air ______ you usually have to wait for hours before the plane takes off.
Hurry up, or you ______ catch the train.
You can improve your English ______ practicing more.