C 本句是被动句式.其主动句式是:It is known that some of the protein chemicals in cells change with age and become less elastic.意为:大家知道细胞内有些蛋白质随着年龄的增大而改变.变得没有那么有弹性.又如:It is known that a new hospital will be built here. = A new hospital is known to be built here.在叙述一般性事物时.动词know之后的不定式不用进行式和完成式. 【查看更多】
