A:Z为0.3mol/L
B:Y2为0.4mol/L
C:X2为0.2mol/L
D:Z为0.4mol/L
小明的发言声音太小,老师要他大声重复一次,老师是要求小明提高声音的( )
900÷____=____
____÷30=____
Fire is very important ________ we need to use it to cook food.