Airport x-ray devices _______ designed more carefully in order to detect the more sophisticated forms of explosive materials that are now produced.
A. must have been
B. must
C. must be
D. must have been being
Answer:C
Airport x-ray devices _______ designed more carefully in order to detect the more sophisticated forms of explosive materials that are now produced.
A. must have been B. must C. must be D. must have been being