题目内容：

根据短文内容，从短文后的选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。

During a research experiment a marine(海洋) biologist placed a shark into a large holding tank and then released several small fish into the tank. 1._______

The marine biologist then inserted a strong piece of clear fiberglass（玻璃纤维）into the tank, creating two separate parts. She then put the shark on one side of the fiberglass and a new set of small fish on the other.

Again, the shark quickly attacked. This time, however, the shark hit the fiberglass divider and bounced off. 2._______ Meanwhile, the small fish swam around unharmed in the second part. Eventually, about an hour into the experiment, the shark gave up.
This experiment was repeated several dozen times over the next few weeks. 3._____  Eventually the shark got tired of hitting the fiberglass divider and simply stopped attacking altogether.

The marine biologist then removed the fiberglass divider, but the shark didn’t attack. 4._____ So, they swam wherever they wished, free from harm.

Like the shark in the story, many of us, after experiencing setbacks and failures, emotionally give up and stop trying. 5.____ In other words, we continue to see a barrier in our heads, even when no ‘real’ barrier exists between where we are and where we want to go.

A. At the same time, the shark tried again and again.

B. We believe that because we were unsuccessful in the past, we will always be unsuccessful.

C. The shark was trained to believe a barrier existed between it and the small fish.

D. It kept repeating this behavior every few minutes fruitlessly.

E. We should keep on trying even if we meet obstacles.

F. Each time, the shark got less aggressive and made fewer attempts to attack the small fish.

G. As you would expect, the shark quickly swam around the tank, attacked and ate the smaller fish.