One suggestion often proferred is that the melanin in black skin,by absorbing ultraviolet light which might otherwise damage DNA and cause mutations,protects against skin cancer.Certainly,white-skinned people who move to the tropics are more at risk of such cancers than they would have been had they stayed at home.But critics of this hypothesis point out that most types of skin cancer,specifically the basal-cell and squamous-cell carcinomas that are the commonest varieties of the disease,tend to affect older people (who have already reproduced and are thus,in Darwinian terms,expendable) and are often not lethal anyway.這句話中profer是什么意思?還有 critics of this hypothesis 這個(gè)該怎么翻譯比較好.最后一句括號(hào)里的怎么翻譯啊