This question already has an answer here:
First, this question may have been partly answered here but what I will ask is very specific.
I have a chain with 3 nodes (A, B and C), all are mining.
A mines the block 1000 and synchronizes with C but not with B. B ALSO mines the block 1000 and synchronizes with C but not with A. They mine this block at the very same time.
A and B have the exact same chain from block 0 to block 999.
How will react C? How can it determine what block is the correct one? (as the chain is the exact same length for both A and B)
EDIT: The possible duplicate is true, but the accepted answer is also valid, even though it is not going into the details, for those interested, go THERE