Medical researchers, who have identified a genetic abnormality in parents of children with Down's syndrome,which they believe, instead of the age of the mother, may be the cause of this congenital birth defect.
(A) Which they believe, instead of the age of the mother, may be --- First, this is a fragment. There is no verb for the subject searchers. Second, 'they' could refer to the researchers or the parents or in a weird way the children also
(B) Which, they believe, may be more important
...
(A) Which they believe, instead of the age of the mother, may be --- First, this is a fragment. There is no verb for the subject searchers. Second, 'they' could refer to the researchers or the parents or in a weird way the children also
(B) Which, they believe, may be more important
...
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