A:NH3、H2O
B:SO2、SCl2
C:PCl3、SO3
D:CH4、C60
Parents will __________ their children __________ making friends when they reach fourteen or fifteen.
His grandfather lives in a small .There are only 200 people in it.
通电直导线周围的磁感线分布如图所示,磁感线是以直导线为中心的一系列的____,从图中可看出,磁感线是____(闭合/不闭合)的曲线;图中小磁针涂黑的磁极是____极.