A:标准状况下,22.4L H2O含有的分子数为1NA
B:常温常压下,1.06g Na2CO3含有的Na+离子数为0.02NA
C:通常状况下,1NA个CO2分子占有的体积为22.4L
D:物质的量浓度为0.5mol/L的MgCl2溶液中,含有Cl﹣个数为1NA
To keep children away from danger,we warn parents ________ children at home alone.
(I)求证:MB∥平面PAD;
(II)求二面角P﹣BC﹣D的余弦值.