Scientists have made a major in cancer research. A. break-inB. breakthrough C. breakupD.

问题补充：

Scientists have made a major in cancer research.

A. break-in

B. breakthrough

C. breakup

D. breakdown

答案：

A. break-in

B. breakthrough

C. breakup

D. breakdown

解析：句意为“科学家们在癌症研究方面取得了一次重大突破。”。make a breakthrough表示“取得突破”，为固定搭配。