The Assumption of Proportional Hazards When Using the Kaplan-Meier Method

The assumption of proportional hazards when using the Kaplan-Meier method states which of the following?

A) The hazard ratio between the two groups must be normally distributed.
B) The hazard ratio between the two groups stays constant over time.
C) The hazard ratio is in direct proportion to the sample sizes in each group.
D) The hazard ratio is the same as the overall number of deaths in each group.