____some doubt among scientists about the cause of cancer.
A．It existsB．There exists C．There is existingD．It is existing
B
考查特殊句式。There be結構中的系動詞換成其它的系動詞如exist, happen，seem等；系動詞是沒有進行時的。所以排除CD。句意：在科學家中對于癌癥的原因存在著一些懷疑和爭論。
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