1.We will put off the picnic until next week,__ we won't be so busy .
A.when b.which c.in which d. where
2.Only those __ knew well could be enrolled in the course .
a.who b.he c.that d.people
3.__ parents say has a life-long effect on their children .
A.that b.which c.what d.as

1 A
next week作先行词，在从句中作时间状语。

2 B
Those是先行词，在从句中作宾语，he是主语，know是及物动词，后面必须接宾语。不能选A，否则knew就没有宾语了。

3 C
这个句子不是定语从句，而是一个主语从句，从句中缺少及物动词say的且之物的宾语，所以选C。