S2 문장의 [______]에 들어갈 단어는?
난이도 필수 · 힌트를 덜 열수록 높은 점수 (현재 5점)
We're creatures who live and die by the energy stores we've built up in our bodies.
Navigating the world is a difficult job that requires moving around and using a lot of brainpower ― an energy-expensive [______].