In Order for a Binomial Tree to Recombine, the Following $u$

In order for a binomial tree to recombine, the following condition is necessary and sufficient:

A) The time-step in the tree must remain constant over time.
B) The up shift factor $u$ and down shift factor $d$ must be such that $u \times d = 1$ .
C) The up shift factor $u$ and down shift factor $d$ must be such that $u \times d =$ a constant.
D) The rate of interest must be zero.