deseada is a small island which belongs to the lesser antilles. this island is said to have obtained its name from the desire christopher columbus felt of seeing land on his second voyage in 1493. it is twelve miles in length and six miles in width. the part which looks to the north is lower than (a) [it / that] which looks to the south. the island abounds greatly in iguanas, and in a species of birds called fragatas. there is a deep cavern on the island, containing the bones and arms of the indians, who, it is supposed, (b) [was / were] buried there. in 1762, this island was taken by the english, (c) [who / where] restored it the following year to the french by the peace of paris, and since that time it has been in the possession of the latter.

(a)에서 it과 that 중 that이 답인데 it은 어째서 답이 될수 없는걸까요? it도 지시대명사로 사용가능한대 말이죠;;