单选题 Most of us go through life adding______to knowledge, polishing a concept here or there, doing an experiment, contributing a few leaves—or, if we are lucky, a twig—to the tree of knowledge.
【正确答案】 C
【答案解析】解析：A．impartially公平地，无私地；B．impassably不可逾越的，不能通行地；C．incrementally递增地，增加地；D．melodiously旋律美妙地。