—This wall of air is an effective answer to the problem  and it ____ work.  —But it is a question to put such a wall of air around the statue. 
#89204
A
may
B
must
C
should
D
could
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 He promised to do what he ______ sure of _______.
#89211
A
could make; my happy
B
could to make; me happy
C
could make; my happiness
D
could to make; my happiness