A:次氯酸钠溶液中加入双氧水有氧气放出:ClO—+ H2O2=O2↑+ Cl—+ H2O
B:碳酸钙溶于稀醋酸溶液:CaCO3+ 2H+= Ca2+ + CO2↑+ H2O
C:用稀硝酸洗涤做过银镜反应的试管:Ag + 4H+ +NO3—= Ag+ + NO↑+2H2O
D:苯酚钠溶液中通入少量CO2:2 + CO2 + H2O → 2+ CO32-
4与9的等比中项是____ .
My father told me ________ football in the street. It's dangerous.