The professor, his assistants, conducting the experiment.
A. in addition, is
B. as well, is
C. in addition to, are
D. as well as, is
网友回答
A. in addition, is
B. as well, is
C. in addition to, are
(答案→)D. as well as, is
解析：考查固定搭配和主谓一致。In addition另外（单独使用，不接宾语）；as well也（单独使用，不接宾语）；in addition to除…之外（介词短语）；as well as和…（介词短语）；句意：教授和他的助手正在做实验。“名词A+as well as/tother with/in addition to+名词B”做主语的时候，谓语动词要和名词A保持一致。故D正确。