______ valuable experience, he asked to be sent to remote areas. [      ]

A. To gain    

B. Gaining    

C. Gained        

D. Being gained

质量分数为a%，物质的量浓度为c mol/L 的KOH溶液，蒸发溶剂，恢复到原来温度，若质量分数变为2a%，则物质的量浓度变为                                 

A．大于2c mol/L

B．等于2c mol/L

C．小于2c mol/L

D．在c mol/L-2c mol/L之间