Clusters of brain cells has allowed scientists to discover the functions of various parts of the brain.

The answer is tiny, it is because the clusters in the brain which are tiny, morphologically, is helpful for scientist in discovering the brain's functions and in the same time, it does not harm the individual involved with the experiment or the study that is being conducted.

The correct answer is option (B) O-.

Blood groups are classified into four groups called the A, B, AB and O based on the presence or absence of antigens A and B. These are the glycoproteins present on the membrane of the red blood cells. Apart from this another antigen called the Rhesus factor (Rh) classifies each group into positive or negative. The presence of this antigen makes the blood groups A+, B+, AB+ and O+ and the absence of this antigen makes the blood groups A-, B-, AB- and O-.

The blood groups and the antigens on them are mentioned below.

Blood group             Antigen                           Rh positive                   Rh negative

A                                  A                                          A+                                 A-

B                                  B                                          B+                                 B-

AB                          Both A and B                           AB+                               AB-

O                            Neither A nor B.                       O+                                 O-

Since, O- is the blood group without the antigens A, B and Rh factor, the blood can be donated to the other three blood groups. Thus, O- is called the universal blood donor as it is compatible with any bloof type.