'while' is a conjunction,and it is used to show contrast. The first subject compared is 'rate of additon....', so the second subject compared should be the counterpart of the first subject. Among the answers, only D meets the requirement. 'The rate of loss' is parallel to 'the rate of addition....'. As for the parallelism between the two verbs, it is less important. If the 'rate' were to be replaced by a pronoun, the pronoun would be 'that' rather than 'those'. Thus, A is wrong.