The doctor must be _______ 24 hours a day, 7 days a week in the boarding school. 
#25574
A
on call
B
by hand
C
in sight
D
on spot
推荐题目
Many chemical substances react________oxygen. 
#25154
A
on
B
with
C
against
D
to
Do you know that girl ?The one ___red.
#28139
A
having
B
wearing
C
in
D
putting on
   ______hearing the news that a hurricane was______, most of the people hurried to safer places.
#26342
A
In, on the way
B
Upon, on the way
C
On, in the way
D
As soon as, in this way