问题补充:
He doesn’t know they will plant trees on Saturday or not.
A. weather
B. whether
C. if
D. that
答案:
A. weather
(答案→)B. whether
C. if
D. that
解析: weather天气;whether是否; if是否,如果;that因为,以至于.句意:他不知道他们是否在周六植树.if和whether都可以表示是否的含义,但是只有whether和or not搭配使用,故选B.