问题

All the students ______ their study _______ all the lights went out.
A:were paying their attention on when
B:were fixing their attention on when
C:were fixing their attention to whileDhad been focused on while
D:had been focused on while
B:were fixing their attention on when
C:were fixing their attention to whileDhad been focused on while
D:had been focused on while
正确答案是: