If you add a drop of anti-A serum and it clumps the blood type has blood type B markers that will fight off the A serum making the blood type B....
I got this because...
there is A, B, AB, and O blood
A has anti-B clumping proteins
B has anti-A clumping proteins
AB has no clumping proteins
O has anti-A, anti-B, and anti-AB clumping proteins
so the only on one with anti-A clumping proteins is blood type B
hope this helps :)