The manager was concerned to hear that two of his trusted workers  ________.[ ]

A. will leave  

B. are leaving

C. have left  

D. were leaving

物质是由分子组成的，分子是由原子构成的，原子是由      和原子核构成的，其中       带负电。原子核是由      和      构成，其中     带正电，    不带电。通常情况下，原子对外       （选填“不显”或“显”）电性。