Scientists have sought evidence of long-term solar
periodicities by examining indirect climatological data,
such as fossil records of the thickness of ancient tree rings.
These studies, however, failed to link unequivocally
terrestrial climate and the solar-activity cycle, or even to
confirm the cycle's past existence.
It can be inferred from the passage that studies attempting to use tree-ring thickness to locate possible links between solar periodicity and terrestrial climate are based on which of the following assumptions?
答案：D