There are currently two models explaining_______________________________________.

 The more widely accepted explication, called the big bang theory, holds that the universe 

 started in a highly compressed primordial state with a fixed amount of matter, which

 started to expand 13.7 billion years ago and is still expanding today. An alternative model,

 the steady state theory, posits that the universe has no beginning or end. Proponents of 

 this theory claim that the universe will expand forever, but new matter will be continuously

 created to keep the density of the cosmos constant.

 (a) why the universe is always expanding

 (b) the supposed shape of the universe

 (c) when the universe will cease to exist

 (d) the nature of the known universe

 저는 답을 (a) why the universe is always expanding으로 했는데요. 그 근거가 뒤에 나오는 big bang theory와, alternative model이 universe의 시작이나 끝, 그리고 확장에 대해서 설명한다고 이해했기 때문입니다. 그런데 답은 (D)이구요. 해설지에는 big bang theory와 alternative model 둘 다 nature of the known universe라는데, 이해가 가지 않아서 논리적으로 설명해주실 고수분 구합니다.