글의 흐름으로 보아 빈 칸 (A), (B)에 들어가기에 알 맞은 표현으… | [법원직 9급 영어] 07년 1회차 기출문제 상세 페이지

글의 흐름으로 보아 빈 칸 (A), (B)에 들어가기에 알  맞은 표현으로 짝지은 것은?
The dolphin has an echo system that works while it sleeps. This system sends out clicking sounds every 15 or 20 seconds. The sounds bounce back from anything in the water and tell the dolphin what is nearby. Some dolphins   (A)  open and close their eyes while they sleep. They may do this to watch for enemies in the open sea. Of course, the clever dolphins may also just be watching the people who are watching them. But the dolphin's restless way of sleeping lets it breathe and  (B) .

(A) have been seen to (B) keep it safe

(A) have been seen to (B) keeps it safe

(A) have been seen (B) keep it safe

(A) have been seen (B) keeps it safe