Given the task definition, example input & output, solve the new input case.
In this task, you are given a sentence. You are expected to write the name of the organism substances in the body mentioned in the input. Although there might be several correct answers, you need to write one of them.
Example: Antisperm antibodies were assessed in the serum samples of 73 men unable to ejaculate naturally and on the sperm cells of 13 of these men.
Output: serum
Serum is a substance of the organism and needs to be tagged.

New input case for you: A 20 - mul aliquot of the supernatant was added to 12 . 5 mul of assay buffer ( 20 mm ATP , 40 mm MgCl2 , 0 . 4 m Tris - Cl , pH 7 . 8 ) .
Output:
supernatant