The suggested causes of a given phenomenon cannot always be independently observed, and so it is hard to ______ the possibility of there being explanations alternative to the one proposed.

单项选择题There are three bodies of writing that ______ on this question and we will consider each in turn.

单项选择题Children who are described as mildly mentally handicapped are often upset to hear themselves described as such, and such terms as children with learning difficulties are now ______.