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 the assumption that ______.
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