Teacher: In this task, you will be given sentences in which your task is to recognize the name of the organ of the body. An organ is a collection of tissues that structurally form a functional unit specialized to perform a particular function. Although there might be several correct answers, you need to write one of them.
Teacher: Now, understand the problem? If you are still confused, see the following example:
I have serious allergy in my left eye.
Solution: eye
Reason: Eye is an organ which has been affected, hence should be tagged.

Now, solve this instance:  We plan to systematically investigate the effects of thawing and refreezing on RNA integrity , by deliberately thawing and freezing small samples of brain tissue over varying time intervals and temperatures .
Student:
brain