some doubt among scientists about the cause of cancer.
A. It exists
B. There exists
C. There is existing
D. It is existing
网友回答
A. It exists
(答案→)B. There exists
C. There is existing
D. It is existing
解析：考查特殊句式。There be结构中的系动词换成其它的系动词如exist, happen，seem等；系动词是没有进行时的。所以排除CD。句意：在科学家中对于癌症的原因存在着一些怀疑和争论。