根据下面资料 回答

根据下面资料，回答
By the time I finished high school, my interest in animals had grown, and I enrolled at a university to study biology. I learned soon enough that studying animals at this level was not in the animals′ best 31 . I remember one midterm exam in 32 each student was handed a large, freshly-killed frog and instructed to dissect (解剖) and mark a set of body parts. I looked at the 33 frog in front of me and was saddened that her life was 34 away for such a slight reason.
A year later, in the same lab where I dissected the flog, I performed a small act of animal operation. We were 35 on fruit flies, and it was time to record the distribution of characteristics in their next generation. Flies were kept in small plastic bottles. Counting the number of flies with white or red eyes required first exposing them to ether (乙醚) 36 they could not move. The flies were then spread onto a piece of white paper 37 and counted. When the data collection was complete, the flies had no further use, and our instructions were to put them into a small glass dish of oil at the center of each desk, which was to be their final resting 38 .
Once my little pile of flies had been counted, I pushed them off the edge of the paper. As we recorded our data, I kept one eye 39 them. Within minutes the pile was humming (嗡嗡叫) as tiny legs and wings beat their way out of the ether fog. I was extremely excited as they took flight. That was my first step in refusing to conduct scientific research that treated nonhuman life in a (n) 40 way.
第（39）题选
A.for
B.with
C.at
D.on

答案：D解析：考查固定搭配。keep one eye on sth表示“留意某物”。故选D。