The ________ working age in western countries is usually 14 to 16 , depending on the number of hours

and type of work. [ ]

A. regular  

B. legal      

C. secure  

D. logical

下列各组物质的溶液混合后，能发生反应，且反应前后溶液的总质量不变的是 

A．NaOH和HCl

B．KNO₃和NaOH

C．Ba(NO₃)₂和Na₂SO₄

D．NaHCO₃和HCI