—What's behind the door? —There must be
题型:选择题
问题:
—What's behind the door?
—There must be a dog _____ behind it. [ ]
A. to sit
B. sit
C. sitting
D. to sitting
—What's behind the door?
—There must be a dog _____ behind it. [ ]
A. to sit
B. sit
C. sitting
D. to sitting