21.a)to be on the alert when being followed.
b)not to leave senior citizens alone at home.
c)not to let anyone in without an appointment.
d)to watch out for those from the electricity board.
答案:C
21.a)to be on the alert when being followed.
b)not to leave senior citizens alone at home.
c)not to let anyone in without an appointment.
d)to watch out for those from the electricity board.
答案:C
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