When H7N9 bird flu appeared, people didn't know __________ doctors could cure it.
A：if
B：that
C：what
D：why
抛物线y＝－(x＋2)2－3的顶点坐标是（ ）．
进入青春期后，我们与父母的冲突，往往基于父母对我们的高期待、严要求。要化解这种“爱的冲突”，我们应该（ ）
