Complete the following sentence: The concern with the "self-strengthening" of China was expressed by Feng Guifen (1809<unk>1874) in a series of essays presented by him to Zeng Guofan in 1861. <unk> <unk> <unk> <unk> <unk> <unk> <unk>
Taiping rebels. ️<unk>️<unk>️<unk>️