Why is it said that the underclass is trapped?

Several climate feedback loops to increase temperatures from the last glacial 20,000 years ago to interglacial conditions today. which of the below is a negative feedback loop rather than a positive feedback loop? a. warming temperatures and more rainfall globally at the end of the last glacial period caused increased growth of vegetation which would act to decrease atmospheric carbon dioxide levels. b. warming of the oceans at the end of the last glacial period releases carbon dioxide to the atmosphere (because less carbon dioxide can dissolve in warmer water). c. warming temperatures at the end of the last glacial period resulted in thawing of permafrost over large areas which released carbon dioxide to the atmosphere. d. warming temperatures at the end of the last glacial period caused a reduction in ice covered areas (sea ice, ice sheets and snow) which decreased the earth's albedo.