Professor Smith, along with his assistants, _____ on the project day and night to meet the deadline.
#101966
A
work
B
working
C
is working
D
are working
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The doctors have recommended that she _______ on, but she has not yet made up her mind.
#103553
A
will be operated
B
is operated
C
be operated
D
was operated
 I know Mr. Brown; we ______ to each other at an international conference.
#104101
A
are introduced
B
are been introduced
C
were introduced
D
had been introduced
Experiments of this kind ___in both the U.S and Europe well before the Second World War.
#100251
A
had conducted
B
have conducted
C
have been conducted
D
had been conducted
Helen ______ her keys in the office so she had to wait until her husband ______ home. 
#102080
A
has left; comes
B
left; had come
C
had left; came
D
had left; would come