1 . I never forget this summer vacation, ___________I spent the happy days with my friends.

2 . According to medical experts, there is a“golden period", the first 4 minutes of a heart attack,____________CPR(心肺复苏) is most effective in saving the persons' life. (用适当的词填空)

3 . I didn't become a serious climber until the fifth grade，________ I went up to rescue a kite that was stuck in the branches of a tree. (适当形式填空)

4 . I didn't become a serious climber until the fifth grade, _____ I went up to rescue a kite that was stuck in the branches of a tree.

A．when	B．where
C．which	D．why

5 . Between the two parts of the concert is an interval, ________ the audience can buy ice­cream.

A．when	B．where
C．that	D．which

6 . The book was written in 1946, ____ the education system has witnessed great changes.

A．when

B．during which

C．since then

D．since when