矿井中容易造成有害气体的急性或慢性中毒，以（）等最为明显。
A、CO
B、SO2
C、NO2
D、CO2
参考答案：A, B, C
You can’t imagine ________ chatting online is!
A．how a great fun
B．how great fun
C．what a great fun
D．what great fun
To test eggs , ___them in a bowl of water: if they float they are bad; if they sink they are good.
A．put
B．putting
C．to put
D．to be putting
