밑줄 친 부분에 들어갈 말로 가장 적절한 것을 고르시오.A group … | [9급 국가직 영어] 16년 1회차 기출문제 상세 페이지

밑줄 친 부분에 들어갈 말로 가장 적절한 것을 고르시오.
A group of tribes and genera of hopping reptiles, small creatures of the dinosaur type, seem to have been pushed by competition and the pursuit of their enemies towards the alternatives of extinction or adaptation to colder conditions in the higher hills or by the sea. Among these distressed tribes there was developed a new type of scale—scales that were elongated into quill-like forms and that presently branched into the crude beginnings of feathers. These quill-like scales lay over one another and formed a heat-retaining covering more efficient than any reptilian covering that had hitherto existed. So they permitted an invasion of colder regions that were otherwise uninhabited. Perhaps simultaneously with these changes there arose in these creatures a greater solicitude for their eggs. Most reptiles are apparently quite careless about their eggs, which are left for sun and season to hatch. But some of the varieties upon this new branch of the tree of life were acquiring a habit of guarding their eggs and __________. With these adaptations to cold, other internal modifications were going on that made these creatures, the primitive birds, warm-blooded and independent of basking.

hatching them unsuccessfully

leaving them under the sun on their own

keeping them warm with the warmth of their bodies

flying them to scaled reptiles

빈칸에는 "keeping them warm with the warmth of their bodies"(자신의 체온으로 알을 따뜻하게 하는 것)가 들어가야 한다.
이 생물들에게 알에 대한 "a greater solicitude"(더 큰 배려)가 생겼고, 알을 "left for sun and season to hatch"(햇볕과 계절에 맡겨 부화시키는) 대부분의 파충류와 대조되며, 뒤의 "With these adaptations to cold"(추위에 대한 이러한 적응)로 이어진다.
"hatching them unsuccessfully": 알을 보살피는 적응이라는 긍정적 흐름과 맞지 않는다.
"leaving them under the sun on their own": 대부분의 파충류가 하는 방식으로, "But"으로 대조되는 내용과 반대다.
"flying them to scaled reptiles": 문맥상 의미가 통하지 않는다.