While ______ a tunnel through the mountain, the workers discovered an underground lake.
A. built
B. to built
C. being built
D. building
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Do you know the girl ______ by the window?
A. sits
B. to sit
C. sitting
D. sat
The police searched the area and finally found the ______ boy.
A. missing
B. missed
C. to miss
D. miss
The ______ waiter came to us and said "You're welcome".
A. smile
B. smiling
C. to smile
D. smiled
Don't sit there ______ nothing.
A. do
B. done
C. doing
D. to do