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? A. The solar-activity cycle existed in its present form during the time period in which the tree rings grew. B. The biological mechanisms causing tree growth are unaffected by short-term weather patterns. C. Average tree-ring thickness varies from species to species. D. Tree-ring thicknesses reflect changes in terrestrial climate. E. Both terrestrial climate and the solar-activity cycle randomly affect tree-ring thickness.