S1 문장의 [______]에 들어갈 단어는?

난이도 필수 · 힌트를 덜 열수록 높은 점수 (현재 5점)
In 1890, William James described attention as "the taking possession by the mind, in clear and vivid form, of one out of what [______] several simultaneously possible objects or trains of thought."