Which of the following blood groups' person cannot donate blood to others?

Erythroblastosis foetalis can be prevented in subsequent pregnancies of $Rh^+$ husband and $Rh^-$ wife by:

Leucocytes $(WBCs)$ are considered as true cells because

Serum differs from blood in

Choose the incorrect sentence$(s)$.
$(I)$ $RBCs$ are formed in the white bone marrow in adults.
$(II)$ $RBCs$ have a red colour because of the presence of a protein called haemoglobin.
$(III)$ The nucleus is present in $RBCs$ in most mammals,and they are biconcave in shape.
$(IV)$ $A$ healthy individual has $12-16 \, g$ of haemoglobin in every $100 \, ml$ of blood.

Blood = $.....$