题目

It is sometimes said that strange flying objects have been seen high up in the sky. People call these strange objects Unidentified Flying Objects（UFOS）. These UFOs caused a lot of interest. Some of the reports about them are difficult to believe. Some have been explained in scientific ways. Others have never been explained. Here is an account of UFOs from a report. Dennis Burdens, who wrote an article, tells us that he once saw a UFO himself. He was having dinner one night with a friend near Tripoli, Libya, and after dinner the two men walked across a yard to his room to look at some paper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