(Each chemical)A element is characterized (to)B the number of protons that (an atom)C of that element contains,called (its)D atomic number.
热心网友
选Ｂ，to应该为byEach chemical element is characterized by the number of protons that an atom of that element contains,called its atomic number.其中（the number of protons ）that an atom of that element contains是定语从句，意思：该种元素的一个原子所包含的（质子数），an atom of that element 在从句中作主语，故Ｃ是对的．